Interlamellar Amorphous Phase Research Articles

Retrospecting the relationship between interlamellar amorphous structure and intrinsic modulus of semicrystalline polymers via in-situ positron annihilation lifetime spectroscopy and in-situ X-ray scattering

The structure-property relationship of the interlamellar amorphous phase in semicrystalline polymers has been a fundamental yet controversial topic in polymer physics for several decades. The intricate hierarchical structure and limited availability of experimental techniques pose significant challenges in quantitatively characterizing the evolution of the amorphous structure and establishing its connection to the mechanical behavior. In this work, the unannealed and annealed isotactic polypropylene hard-elastic films composed of idealized series-arranged lamellar stacks are studied to simplify the mechanical coupling between the two phases. By utilizing the synchrotron-based in-situ small and wide-angle X-ray scattering (SAXS/WAXS) techniques, the apparent modulus and overall volumetric strain of the amorphous phase are estimated. Moreover, with our significant progress in the counting rate of positron annihilation lifetime spectroscopy (PALS), the minute-scale structure evolution of amorphous phase (free-volume pores) during the tensile process is now feasible to be captured. Combing in-situ X-ray scattering and in-situ PALS results, in the elastic regime, it is found that triaxial stress in the amorphous phase leads to the expansion of free-volume pores (about 23% volume increase for the annealed sample, 35% volumetric strain for the unannealed one) and density decrement within occupied volume by overcoming Van Der Waals force. Our quantitative analysis also reveals that the annealing process not only enhances the crystallinity and aspect ratio of lamellae but also weakens the intrinsic modulus and chain packing density of amorphous phase.

Elastic modulus of the amorphous phase confined between lamellae: The role of crystalline component

The correlation between the modulus of the interlamellar amorphous phase and the microstructure of the crystalline component was studied. The experimental method involving deformation of the amorphous layers only developed by us was used. Three polyethylenes with different architecture of macromolecules and thermal history were analyzed. In the case of HDPE and low density polyethylene (LDPE), a practically linear increase of the elastic modulus of the confined amorphous phase with the increase of the crystal thickness (in the range of ≈4–20 nm) was observed and explained by activation of α relaxation process in crystalline lamellae. For thin crystals the degree of activation of the α relaxation was intense and the influence of crystalline component on the modulus of amorphous regions was the lowest. For materials with thick crystals the degree of activation of the α relaxation was lower and the “confining” influence of crystalline component on the amorphous phase was the highest. In ethylene-octane copolymer (EOC) materials with the thinnest crystals (≈2–3 nm) the modulus of the confined amorphous phase was close to the modulus of the bulk amorphous phase-in those materials the α relaxation process was not observed at all. The modulus of the amorphous phase of isothermally crystallized HDPE (≈70 MPa) was over twenty times higher than the modulus of the bulk amorphous phase (≈3 MPa).

Effect of molecular weight on crystallization behavior of poly (lactic acid) under isotherm and non‐isotherm conditions

AbstractThe effect of molecular weight on the crystallization behavior of poly(lactic acid) (PLA) both isothermally and non‐isothermally was studied using differential scanning calorimetry and polarized optical microscopy. Two distinct crystal forms (α and α′) were investigated under normal conditions of crystallization for different molecular weights. Using microscopy, the growth rate of spherulites was measured and the nucleation rate was estimated. Heterogeneous nucleation and morphology with a variant α and α′ forms have been proposed. Spherulites from low molecular mass PLA contained more interlamellar amorphous phase. In addition, the results showed more rapid crystallization of α′ than the α form in isothermal conditions.

The Modulus of the Amorphous Phase of Semicrystalline Polymers

A universal method for determining the modulus of the interlamellar amorphous phase of semicrystalline polymers was proposed. The basics of the method were presented on the example of high-density polyethylene (HDPE). The local deformation of the amorphous component was induced by the introduction of a swelling agent (hexane). The swelling-induced local strain and local stress of the interlamellar amorphous phase were estimated based on the changes of the long period and the yield stress, respectively. The determined modulus of the interlamellar amorphous phase of HDPE (≈40 MPa) was an order of magnitude higher than the modulus of the bulk rubbery amorphous phase of HDPE (≈3 MPa). The modulus of the interlamellar amorphous phase of other semicrystalline polymers, such as polypropylene (PP), low-density polyethylene (LDPE), and an ethylene–octane copolymer (EOC), was also determined and amounted to 50.3, 11.8, and 4.2 MPa, respectively. For polyethylene materials (HDPE, LDPE, and the EOC), a linear increase in the modulus of the interlamellar amorphous phase with increasing of the degree of crystallinity was observed. This effect was correlated with a decrease in “deformability” of the amorphous layers (a decrease in the swelling-induced local strain of amorphous layers for samples with higher crystallinity). The decrease in “deformability” of the amorphous component at low strains was probably caused by an increase in the density of stress transmitters (entanglements and tie molecules), which affected the mechanical properties (modulus) of the interlamellar regions. In the case of the material with the lowest degree of crystallinity (EOC, 26.2 vol%), the modulus of the interlamellar amorphous phase was similar to the modulus of the bulk rubbery amorphous phase, which indicated a very limited influence of the lamellar crystals on the stiffness of the amorphous regions.

Mechanical Behavior—Microstructure Relationships in Injection-Molded Polyamide 66

Clear relationships between the semi-crystalline microstructure of injection molding polymers and their mechanical behavior are not yet totally established for all polymers. Part of this relative lack of understanding is because an unambiguous scientific approach is difficult to build up. The processing of samples promotes a microstructure which is not uniform and can be described in various ways on different scales. This introduces uncertainty in the correlations. Most completed studies were conducted on polyolefin, which exhibits an evolution of microstructure that is quite easy to observe and to correlate to mechanical properties. This paper intends to illustrate a more diffuse case. To achieve this, combined characterizations along the flow path and throughout the thickness of a plaque as well as characterizations of the local microstructure and tensile behavior of polyamide 66 are described. The microstructure was explored in terms of skin-core structure, spherulites sizes, crystallinity ratio and lamellae organization. Mechanical properties were addressed with non-monotonic tests with the use of DIC (Digital Image Correlation) to assess true behavior. The effect of humidity is also accounted for. It is demonstrated that small changes in lamellae or interlamellar amorphous phase are likely to be responsible for non-uniform mechanical properties, whereas more macroscopic levels (skin core structure, spherulites level of crystallinity ratio) appeared to be irrelevant levels of description. Consequently, the usual simplified analyses based on optical microscopy and differential scanning calorimetry (DSC) can be inefficient in improving knowledge in that field.

Stress-induced microphase separation of interlamellar amorphous phase in hard-elastic isotactic polypropylene film

Lamellar stacks, arranged alternatively with hard crystal and soft amorphous nano-layers, are the basic building units to determine the intrinsic mechanical properties of semi-crystalline polymeric materials. The mechanical instability of hard-elastic isotactic polypropylene (iPP) films with highly parallel lamellar stacks is studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during cyclic tensile deformation. Unexpectedly, the micro-strain, deduced from the relative variation of lamellar periodicity, shows an accelerated increase at the onset of instability and reaches values larger than the corresponding macro-strain after yield, during which no irreversible plastic destruction of crystal is observed. Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXS measurements, we propose that stress-induced microphase separation of interlamellar amorphous phase is responsible for the yield behavior and the hyperelasticity of hard-elastic iPP films, which stems from the heterogeneous distribution of tie chain/trapped entanglement in interlamellar amorphous nano-layers. This reversible stress-induced non-equilibrium phase transition of interlamellar amorphous phase is different from current plastic deformation models with crystal destruction in semi-crystalline polymers but in line with the nearly 100% elastic recovery ratio of hard-elastic films.

Yield behavior of random copolymers of isotactic polypropylene

The crystallographic micromechanical model (CMM) for prediction of yield stress of semicrystalline polymers, based on the thermally activated nucleation of screw dislocations at the boundary of lamellar crystals, is employed to interpret the yield behavior by effect of uniaxial drawing of some isotactic copolymers of propylene with different comonomeric units such as ethylene, 1-butene, 1-pentene, 1-hexene, and 1-octadecene (iPPEt, iPPBu, iPPPe, iPPHe and iPPOc, respectively). The samples are characterized by a random distribution of the comonomeric units. The CMM predicts that the values of stress at yield depend on the thickness of the lamellar crystals and relies on two parameters, i.e. the critical value of the free energy needed for nucleation and activation of a screw dislocation in crystallographic planes parallel to the chain axes, and the shear modulus relative to the planes of slip for the dislocations, whereas the role of the interlamellar amorphous phase is neglected. The aim of this study is to analyze to which extend the thickness of the lamellar crystals influences the yield stress for a series of propylene-based copolymers having a well-defined chain microstructure and crystallized in the α and/or γ forms, but possessing different degree of crystallinity, thickness of the lamellar crystals, stability of the crystals and also intrinsic flexibility of the portions of chains belonging to the amorphous regions, in relation with the degree of inclusion (exclusion) of the co-monomers in (from) the crystals. It is shown that, in the case of copolymers with a comonomer concentration lower than a threshold, the yield stress increases with increasing the lamellar thickness regardless of type of comonomer, the relative amount of the two polymorphs (α and γ forms), and the degree of inclusion, in good agreement with the predictions of the CMM approach. For copolymers with comonomer concentration higher than a threshold and bulky side groups, which are excluded from the crystals, the thickness of the lamellar crystals becomes low and the role of the chains in the amorphous regions becomes not negligible. In the case of copolymers with high butene concentration, instead, the stress at yield decreases with increasing the thickness of the lamellar crystals, because the high degree of inclusion of butene units in the crystals induces not only an increase in the thickness of the lamellar crystals, but also a decrease in the stability of the crystals, and the role of the chains located in the interlamellar amorphous layers acting as tie-molecules may not be disregarded.

Stretching induced phase separation in poly(vinylidene fluoride)/poly(butylene succinate) blends studied by in-situ X-ray scattering

The structure evolution of poly(vinylidene fluoride)/poly(butylene succinate) (PVDF/PBS) blends during stretching above the melting point of PBS is investigated by synchrotron-based simultaneous wide angle and small angle X-ray scattering (WAXS/SAXS). Before stretching, PVDF crystallizes into the α-form, whereas the chains of molten PBS locate at the inter-lamellar amorphous phase of PVDF. Crystal transition from α to β of PVDF is observed in all samples during stretching. The morphological transformation from a lamellar structure into a fibrillar structure occurs at low and intermediate strains. With further deformation, a “stretching induced phase separation” phenomenon is observed. The final microstructure of PVDF/PBS blends contains PVDF microfibrils with PBS chains preferentially distributed in the inter-fibrillar region. The PBS molecular weight influences the onset and end strain for the transition. A new “two-step model” is proposed to describe the deformation process.

Study of an initial transient relaxation in XLPE cable insulation by TSDC and PEA

Space charge profiles in electrets of cable XLPE insulation have been determined by pulsed electroacustic method (PEA). A single positive peak close to the cathode, which decreases with annealing time, is observed. Typically, thermally stimulated depolarization currents (TSDC) of those electrets show heteropolar peaks initially. A new transient homopolar peak emerges in TSDC profiles at 99°C with annealing at high temperatures. No direct correlation can be established between PEA and TSDC results, since the case of the largest value of the maximum of this TSDC peak corresponds to the smallest charge peak observed in PEA profile. The transient behavior is explained by means of charge trapped on crystals surfaces due to the differences between the conductivities of amorphous and crystalline phases. The homoplar peak is associated with dipolar contaminants located in the interlamellar amorphous phase, which are reoriented by the field produced by charge trapped on crystal surfaces, during the TSDC discharges. Detrapping of charge trapped on crystal surfaces occurs when lamellae melt, which contributes homopolarly to the current. Both contributions result in the peaks observed in the TSDC discharge.

Steady state fluorescence to study nanocomposites based on silica nanoparticles and thermoplastic polymer matrices

Steady state fluorescence was used to study the interfaces of composites formed by silica nanoparticles and three polymers: poly(methylmethacrylate) (PMMA), polystyrene (PS), and low density polyethylene (LDPE). The fluorescent response from the pyrene-1-sulfonamide (PSA) was used to study changes appearing in its immediate surroundings. Molecular dynamics of the polymers was studied monitoring the fluorescent response from the PSA as a function of temperature. When the fluorophore was dispersed within the polymers, information from their bulk was obtained while, attaching the fluorophore to the surface of the silica nanoparticles, information from the interface was collected. In the case of the amorphous polymer matrices (PMMA and PS), the presence of silica nanoparticles exerts a small constrain effect by reducing the chain mobility. In the case of the semicrystalline thermoplastic polymer (LDPE) when nanoparticles are not present, only one clear relaxation assigned to the typical diffusion-like motion of chain segments in the crystallites has been observed. However, when nanoparticles are within the polymer, three relaxations are clearly observed: one in the interlamellar amorphous phase and two due to diffusion-like motion of chain segments in the crystallites under or without the influence of the nanoparticles, respectively. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

A study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

The glass transition of poly(ethylene terephthalate) (PET) crystallized for 4 h at temperatures between 413 and 453 K was studied. Secondary crystallization processes were monitored by differential scanning calorimetry and the glass transition of the remaining interlamellar amorphous phase was studied by thermally stimulated depolarization currents measurements. Non-isothermal window polarization is employed to resolve the relaxation in modes with a well-defined relaxation time that is subsequently adjusted to several standard models. An analysis of experimental results reveals that cooperativity can be disregarded in the modelization of data. The evolution of modes during secondary crystallization, once primary crystallization has been completed, gives more weight to lower energy modes. As a consequence, secondary crystallization tends to lower the glass transition temperature of the amorphous interlamellar phase, although remaining noticeably higher than in amorphous samples. The evolution of calorimetric scans of the glass transition is simulated from the obtained results and shows the same behaviour. Regarding the glass transition temperature of the material, it can be concluded that primary and secondary crystallization act in opposite directions even though the effect of secondary crystallization is much smaller. The interpretation of these results in terms of current views about secondary crystallization is discussed.

Two Lamellar to Fibrillar Transitions in the Tensile Deformation of High-Density Polyethylene

The structural evolution of tensile-deformed high-density polyethylene (HDPE) at the lamellar level was investigated as a function of strain using a scanning synchrotron small-angle X-ray scattering technique. Intralamellar crystalline block slips were activated at small deformations, whereas stress-induced fragmentation and recrystallization process proceeded at a larger strain, yielding lamellae with polymeric chains preferentially oriented along the stretching direction. The critical strains marking the onset of the destruction of original crystallites and the fibril formation for isothermally crystallized HDPE were at about 0.4 and 1.2, respectively. In the case of a quenched sample, the critical strain was 0.4. In the isothermally crystallized sample two critical values were observed that could be traced back to the existence of two populations of lamellar stacks with significantly different interlamellar amorphous phase thicknesses. This resulted in distinct mobilities of the amorphous domains and, therefore, different moduli of the entangled amorphous networks. Consequently, the strain required to produce the critical network stress, which gave rise to a fragmentation of the crystalline blocks, was different for each stack of crystalline lamellae.

Structural interpretation of the strain-rate, temperature and morphology dependence of the yield stress of injection molded semicrystalline polymers

The yield stress of polypropylene specimens with different initial microstructural states is investigated. These latter were obtained by systematic variations of the processing conditions in injection molding, resulting in different laminated skin-core structures. The morphology of the molded specimens was characterized by polarized light microscopy (full specimen), differential scanning microscopy (core layer) and wide-angle X-ray scattering (skin region). The yield stress was evaluated at different nominal strain-rates (1.67×10 −3 to 150 s −1) and temperatures (23, 40 and 60 °C). The experimental results are analyzed in the frame of Eyring's viscous flow and lamellar cluster models, being established the relationships between the activation volume and enthalpy upon the initial morphological state of the specimens. It is proposed that similar deformation mechanisms operate in both skin and core layers, although with distinct temperature and strain-rate sensitivities. The morphology dependence of the yield stress at different temperatures and strain-rates is established in terms of a laminate composite approach. Yielding is interpreted based on the deformation of crystalline lamellae by the pulling out action of both the molecular chains that are anchored in the inter-lamellar amorphous phase and the tie-molecules. The deformation mechanism operating at the skin and core layers are unified by an elastic-beam mechanical analogue that is able of explaining the morphology, temperature and strain-rate dependences of the yield stress.

Effects of the addition of a graft copolymer of propylene with styrene on thermal behavior, crystallization, and morphology of isotactic polypropylene

A novel graft copolymer of unsaturated propylene with styrene (uPP-g-PS) was added to isotactic polypropylene (iPP) to investigate the state of mixing between uPP-g-PS and iPP phase and effects of the uPP-g-PS addition on iPP intrinsic phase structure, i.e., on iPP spherulitic texture and inner structure of the spherulites fibrillae. Thin films of iPP/uPP-g-PS blends containing 2, 5, and 10% (wt/wt) of graft copolymer were prepared by o-dichlorobenzene casting, according to the method we used previously to obtain samples of iPP/atactic polystyrene/uPP-g-PS ternary blends (D'Orazio et al. J Appl Polym Sci 1997, 65, 1539; D'Orazio et al. J Appl Polym Sci, 1999, 72, 1429). The variation of tan δ with temperature and differential scanning calorimetry thermograms showed respectively that, regardless of blend composition, a single glass transition temperature and a single apparent melting temperature were found for the iPP phase in the iPP/uPP-g-PS binary systems. Optical microscopy investigation revealed that, with increasing uPP-g-PS content, iPP spherulites become more open and coarse. On the other hand, both dimension and number per unit area of the amorphous interspherulitic contact regions decreased. The long period values of the iPP phase, calculated from the peak position of Lorentz-corrected desmeared small angle X-ray scattering profiles, increased markedly with increasing uPP-g-PS content; such an increase was due to an increase of both average crystalline lamellar thickness and interlamellar amorphous phase. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 553–565, 2000

Effects of the addition of a graft copolymer of propylene with styrene on thermal behavior, crystallization, and morphology of isotactic polypropylene

A novel graft copolymer of unsaturated propylene with styrene (uPP-g-PS) was added to isotactic polypropylene (iPP) to investigate the state of mixing between uPP-g-PS and iPP phase and effects of the uPP-g-PS addition on iPP intrinsic phase structure, i.e., on iPP spherulitic texture and inner structure of the spherulites fibrillae. Thin films of iPP/uPP-g-PS blends containing 2, 5, and 10% (wt/wt) of graft copolymer were prepared by o-dichlorobenzene casting, according to the method we used previously to obtain samples of iPP/atactic polystyrene/uPP-g-PS ternary blends (D'Orazio et al. J Appl Polym Sci 1997, 65, 1539; D'Orazio et al. J Appl Polym Sci, 1999, 72, 1429). The variation of tan δ with temperature and differential scanning calorimetry thermograms showed respectively that, regardless of blend composition, a single glass transition temperature and a single apparent melting temperature were found for the iPP phase in the iPP/uPP-g-PS binary systems. Optical microscopy investigation revealed that, with increasing uPP-g-PS content, iPP spherulites become more open and coarse. On the other hand, both dimension and number per unit area of the amorphous interspherulitic contact regions decreased. The long period values of the iPP phase, calculated from the peak position of Lorentz-corrected desmeared small angle X-ray scattering profiles, increased markedly with increasing uPP-g-PS content; such an increase was due to an increase of both average crystalline lamellar thickness and interlamellar amorphous phase. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 553–565, 2000

Characterization of annealed isotactic polypropylene in the solid state by 2D time-domain1H NMR

Two-dimensional time-domain 1H NMR was used to investigate annealed isotactic polypropylene in the solid phase. The spin–lattice relaxation in the laboratory frame and in the rotating frame were correlated with the shape of the free induction decay to identify and characterize relaxation components over the temperature range −120 to 120 °C. Several phase transitions were observed, and three distinct solid phases, with different chain mobilities, were detected. Two of these phases were identified as regions with different mobilities within the crystalline phase. The third phase was characterized by a high degree of isotropy in molecular motion. This phase, identified as the amorphous phase, appeared as the polymer was heated above a low-temperature (−45 °C) phase transition. All transitions observed at higher temperatures occurred exclusively in this phase. About one-third of the polymer chains reside between crystalline lamellae, whereas the majority form amorphous regions outside fibrils of multilamellar structure. Furthermore, the glass-to-rubber transition, occurring above −15 °C, consists of three stages. During the first stage, between −15 °C and 15 °C, regions with an increased segment mobility (labeled intermediate phase) appear gradually within the amorphous phase. At 15 °C, the intermediate phase consists of ∼10% of the polymer units, or one-third of the polymer units constituting the amorphous phase. Between 15 °C and 25 °C, the intermediate phase increases rapidly to 18%. This is associated with the appearance of semiliquid and liquid regions, likely within the intermediate phase. Polymer chain segments (and possibly entire chains) involved in the liquidlike phases exhibit heterogeneous molecular motion with a correlation frequency higher than 106 Hz. These two stages of glass-to-rubber transition occur within amorphous regions outside multilamellar structures. The third stage of the glass transition, appearing above 70 °C, is associated with the upper glass transition and occurs within the interlamellar amorphous phase. Finally, on a timescale of 100 ms or less, spin diffusion does not couple the amorphous regions outside fibrils with crystalline and amorphous regions within multilamellar fibrils. However, on a timescale of hundreds of milliseconds to seconds, all different regions within isotactic polypropylene are partially coupled. It is proposed that the relative magnitude of the crystalline magnetization, as observed in the T1ρ experiment, is a good measure of polymer crystallinity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2487–2506, 2000

The endothermic "annealing peak" of poly(phenylene sulphide) and poly(ethylene terephthalate)

The appearance of an endothermic annealing peak in semicrystalline poly(phenylene sulphide) and semicrystalline poly(ethylene terephthalate) after annealing at or above the cold-crystallization temperature is investigated by temperature-modulated differential scanning calorimetry, thermo-mechanical analysis and dynamic-mechanical analysis. The results indicate relaxation processes in the interlamellar amorphous phase, which is in a strongly constrained state after cold crystallization. During the annealing treatments rearranging processes take place. These processes result in a separation of the amorphous phase into an interlamellar relaxed and a “pseudo-crystalline” phase.

Effect of vinylacetate content on crystallinity and second‐order transitions in ethylene—vinylacetate copolymers

Thermal characteristics of ethylene—vinylacetate (EVA) copolymers having vinylacetate contents ranging from 5 to 40 w/w % are studied by differential scanning calorimetry. It is first shown that EVA copolymers having a vinylacetate content lower than 30 w/w % obey the Flory and Burfield theories of copolymer crystallisation. The minimum sequence length of CH2 ethylenic entities required to participate in a crystalline lamella is also deduced. One can conclude that EVA copolymers represent cases of “total exclusion” of the noncrystallizable comonomer. Moreover, it is observed that when the vinylacetate content is increased, the relative quantity of polyethylene amorphous phase increases and the degree of crystallinity decreases; whereas the β transition temperature of a characteristic-oriented amorphous phase is kept constant. A phase model of ethylene-vinylacetate copolymers, based on an enrichment process of the interlamellar amorphous phase by polyethylene segments originating from the crystalline phase, at increasing vinylacetate content, is proposed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1903–1912, 1997

Effect of vinylacetate content on crystallinity and second-order transitions in ethylene?vinylacetate copolymers

Thermal characteristics of ethylene—vinylacetate (EVA) copolymers having vinylacetate contents ranging from 5 to 40 w/w % are studied by differential scanning calorimetry. It is first shown that EVA copolymers having a vinylacetate content lower than 30 w/w % obey the Flory and Burfield theories of copolymer crystallisation. The minimum sequence length of CH2 ethylenic entities required to participate in a crystalline lamella is also deduced. One can conclude that EVA copolymers represent cases of “total exclusion” of the noncrystallizable comonomer. Moreover, it is observed that when the vinylacetate content is increased, the relative quantity of polyethylene amorphous phase increases and the degree of crystallinity decreases; whereas the β transition temperature of a characteristic-oriented amorphous phase is kept constant. A phase model of ethylene-vinylacetate copolymers, based on an enrichment process of the interlamellar amorphous phase by polyethylene segments originating from the crystalline phase, at increasing vinylacetate content, is proposed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1903–1912, 1997

Concurrent crystallization and interdiffusion in microlayers of polycarbonate and a copolyester

AbstractConcurrent crystallization and interdiffusion of two miscible polymers, polycarbonate (PC) and a copolyester (KODAR), was studied in microlayers with 657 alternating layers. Minimal mixing of the two components during processing was indicated by separate glass transitions of the PC and KODAR layers. Changes in the glass transitions after the microlayers were annealed above the glass transition of PC were determined by DSC and DMTA. When the annealing temperature was 200°C or higher, interdiffusion was much faster than crystallization and the glass transitions gradually shifted closer together until they merged into a single transition. An analysis based on Fick's law of diffusion described the relationship between the glass transition temperatures and the annealing time. At 195°C, the rates of interdiffusion and crystallization were comparable. Annealing at this temperature resulted in more complex changes in the glass transitions, including the persistence of two transitions after long annealing times. This behavior was understood by considering an interlamellar amorphous phase that was trapped in the spherulites and not available for interdiffusion. Models were developed from the spherulite morphology observed in the optical microscope. Two cases were considered: a continuous layer of impinged spherulites in the PC/KODAR 20/80 (w/w) composition, and isolated spherulites aligned with the center of the KODAR layer in PC/KODAR 40/60 and 60/40. These models satisfactorily reproduced the principal features of concurrent crystallization and interdiffusion.