Subglass Process Research Articles

Effect of Copolymerization in the Dynamics of Poly(trimethylene terephthalate)

The structure-dynamics relationships in random copolymers of poly(trimethylene terephthalate) and poly(neopenthyl terephthalate) (PTT–PNT) have been investigated by means of X-ray scattering and dielectric loss spectroscopy experiments. The results show that the incorporation of neopenthyl terephthalate units into PTT chain decreases the ability to crystallize of the resulting copolymers as compared to that of PTT. The subglass dynamics of the copolymers is characterized by the existence of two processes, β1 and β2. We propose that the high frequency secondary relaxation β1 is associated with the most flexible part of the repeat unit, whereas the low frequency mode β2 has been assigned to the carbon of the ester groups linked to the aromatic ring. On the contrary the poly(neopenthyl terephthalate) (PNT), presents only one subglass process, that we hypothesize to have the same origin of the relaxation of β2 relaxation in PTT. The segmental dynamics for the amorphous specimens, characterized by the α-proces...

Dielectric relaxation in amorphous linear aliphatic copolyesters

In order to contribute to a more extensive body of experimental knowledge concerning the relaxation processes in glass-forming amorphous polymers, broad-band frequency dielectric measurements have been carried out on two examples of a model amorphous polar polymer system. The system consists of a linear random copolyester with one-to-one mole ratios of adipic acid and succinic acid coupled with glycol units. In one example, ethylene glycol is the coupling unit and in the other it is trimethylene glycol The subglass β relaxations as well as the primary α regions were characterized and emphasis was placed on phenomenological description of the coalescence of the two regions with increasing temperature. Each subglass process was found to be complex in the sense of exhibiting two partially resolved components. One component is a typical extremely broad-in-frequency subglass relaxation and the other component is weaker, narrower in frequency and occurs at lower frequency isothermally or higher temperature isochronally. Tentative assignments of the source of the β process components are made. Phenomenological analysis of the α– β coalescence region invoking the Havriliak–Negami and Cole–Cole functions and utilizing numerical regression indicates that the merging process is optimally described as the β process losing relaxation strength and its presence being expressed as increased skewing in the primary relaxation.

Effect of orientation and crystallization on dielectric and mechanical relaxations in uniaxially stretched poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films

Microstructural analysis (crystallinity, orientation) have been performed on stretched and isothermally crystallized poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films. The crystallinity ratios are higher at drawing temperatures below Tg than above Tg, and for similar draw ratios, a higher orientation can be obtained at drawing temperature below Tg than above Tg. The molecular mobility study by dielectric relaxation spectroscopy (DRS) and mechanical relaxation spectroscopy (MRS) was carried out as a function of draw ratio (λ) and drawing temperature (Tdraw). Drawing in glassy state apparently splits the α-relaxation into two components: an α-low component which exhibits a strongly accelerated dynamics but this relaxation process is masked by microstructural rearrangements occurring while heating followed by an α-high relaxation process. The two sub-glass processes (β and β∗) are influenced by the drawing in the glassy state. This can be observed from the increase of relaxation amplitudes and was related to a high disorder in the amorphous phase that is induced by drawing below glass transition temperature. Drawing at 160°C induces the opposite trend associated with crystallization and confinements effects. Differences in viscoelasticity behaviour were found by MRS in tensile mode parallel and perpendicular to stretching direction. As a result, when comparing oriented and crystallized samples with the same crystallinity ratios, a strong effect of morphology on the location and amplitude of the three relaxations of PEN can be found.

Crystallization of Poly(l-lactic acid) Probed with Dielectric Relaxation Spectroscopy

The properties of the amorphous component in poly(l-lactic acid), PLLA, depend on its crystalline structure, where the nanoconfinement effects may be monitored using dielectric relaxation spectroscopy. In this work this technique is used to monitor isothermal crystallization in PLLA by probing the evolution of the loss peak with crystallization time. It was found, for two different molecular weight materials and by crystallization from either the glassy or melt states, that the data could be given by a linear combination of three loss processes, where just their intensities vary: the α-process of the amorphous material, the constrained α-process present in the fully crystallized material, corresponding to the segmental motions of the amorphous phase confined by the crystalline lamellae, and the sub-glass β-relaxation. The appearance of the confined process was detected in the earlier stages of the crystallization, suggesting that the confinement effects are effective during primary crystallization. In this case, the cooperative segmental motions are restricted by the primary lamellae, leading to a dynamics characterized by a broader relaxation time distribution shifted to higher values. It was observed that the crystallization, as investigated by monitoring the evolution of the amorphous phase, is quicker for lower molecular weight PLLA and is slower for the material crystallized from the melt with respect to the cold crystallization. It was found that the confined dynamics is not very dependent on the thermal history prior to the crystallization step. The features of the β-relaxation were found to be similar for amorphous and semicrystalline systems, indicating that the sub-glass process in PLLA is not influenced by the crystalline confinement.

Dielectric and mechanical relaxation behavior in poly(butyl methacrylate) isomers

Dielectric and mechanical spectroscopies have been used to study the dynamics in poly(butyl methacrylate) isomers (namely PnBMA, PiBMA and PtBMA). This analysis has revealed four relaxation processes: δ, γ, β and α and has allowed the observation of the evolution of molecular relaxation phenomena with the lateral chain steric hindrance. The α relaxation is associated with the glass transition of the isomers and depends strongly on the lateral chain structure. The secondary relaxation phenomena are very similar for the three isomers. Arrhenius diagrams have been plotted for the three isomers. The activation parameters have been calculated from Arrhenius equation for the sub-glass processes δ and γ whereas the dynamics are characterized by a Vogel–Fulcher–Tamman law for the α relaxations. With a deconvolution procedure for the α and β processes, a departure from the Arrhenius law of the β process in the vicinity of the merging between the α and β phenomena is observed for each isomer.

Broadband Dielectric Investigation of Amorphous and Semicrystalline l-Lactide/meso-Lactide Copolymers

The dynamics of crystalline and amorphous samples of poly(l-lactide) and three l-lactide/meso-lactide random copolymers were investigated in the frequency domain using broadband dielectric spectroscopy (DRS). Two relaxations, the segmental (α) and subglass (β) processes, were observed in the temperature range between −100 and 105 °C. Modification of the characteristics of the α process, i.e., the relaxation strength (Δe), mean relaxation time (τm), and shape parameters, as a function of crystal content was investigated on “fully crystallized” samples as well as in real-time crystallization experiments. As expected, the strength of the α relaxation was smaller and its distribution broader for materials with higher degrees of crystallinity. The relaxation strength of the α process of the amorphous samples increased with temperature, while that of the crystalline materials changed very little or in the opposite direction with temperature. This behavior can be explained by the existence of a rigid amorphous c...

Dielectric Relaxation in Amorphous Poly(ethylene terephthalate) and Poly(ethylene 2,6-naphthalene dicarboxylate) and Their Copolymers

The dielectric loss spectra of poly(ethylene 2,6-naphthylene dicarboxylate) (PEN) and several copolymers of 2,6-naphthylene dicarboxylic acid and terephthalic acid with ethylene glycol have been studied over a wide range of frequency and temperature. Previously, Ezquerra, Balta-Calleja, and Zachmann have reported the presence, in isochronal temperature scans of dielectric loss, of a subglass process (β*) in PEN homopolymer in addition to the subglass process (β) similar to that in poly(ethylene terephthalate) (PET). In the present work both the β and β* processes in PEN and PET/PEN copolymers are resolved and characterized in isothermal frequency scans. It was found that the lower temperature β loss peak in PEN and the copolymers has a complex or composite frequency domain structure requiring two Cole−Cole processes in its representation. The β* process in the copolymers is found to shift to higher frequency isothermally (or lower temperature isochronally) with increasing terephthalic acid content. Analysis of previous dielectric data for PET shows that its β subglass process is also complex in the log frequency axis but must be represented by three Cole−Cole processes. The two higher frequency components are essentially identical to the two Cole−Cole processes making up the β process in PEN. The third and lowest frequency component is interpreted as of the same origin as the β* process in PEN and the copolymers but shifted to higher frequency isothermally (or lower temperature isochronally) to where it overlaps with the β process.

Local and global dynamics of polylactides.: A dielectric spectroscopy study

Polylactides (PLAs) are a family of degradable plastics having a component of the dipole moment both perpendicular and parallel to the polymer backbone (i.e. is a type-A polymer). We have studied the sub-glass, segmental and global chain dynamics in a series of fully amorphous samples having an L: D ratio of 80:20, with molecular weights in the range: 3.8×10 3< M n<4.7×10 4. Finite size effects have been observed for both the segmental and the sub-glass process. The segmental mode was found to have a steeper temperature and pressure dependence compared to the longest normal mode in accord with findings from other type-A polymers. The molecular weight dependence of the longest normal mode relaxation times ( τ∼ M n 3.1) reflects on the intermediate molecular weight regime ( M e∼6×10 3) from Rouse to entangled chains. These findings are compared with the viscoelastic results on the same system. Agreement in the c 2 value of the WLF equation is found but the scaling of the normalized longest relaxation time derived from the rheological measurements shows a stronger than expected dependence on the molecular weight.

Dielectric relaxation behaviour of poly(ethylene naphthalene 2,6 dicarboxylate) (PEN)

Dielectric spectroscopy has been used to study poly(ethylene naphthalene 2,6 dicarboxylate) (PEN) samples of different morphologies obtained by thermally treating bi-axially stretched PEN films. Neat and thermally treated samples of PEN films have been characterised by differential scanning calorimetry in order to measure the glass transition and melting temperatures as well as the degrees of crystallinity. Dielectric analysis has allowed the observation of the evolution of molecular relaxation phenomena with morphology changes and has revealed three relaxation processes: β-, β *- and α-relaxation (increasing temperature). The β-relaxation is associated with local motions of ester and the β *-relaxation with partially cooperative motions of naphthalene groups. The latter has been shown to be related to the morphology of the materials under study. The α-relaxation associated to the glass transition of PEN corresponds to cooperative motions induced by conformational rearrangements of the main chain and depends also on the morphology of the PEN films. Dielectric relaxation behaviours were compared using the activation energies calculated from Arrhenius equation formalism for the two sub-glass processes. Vogel–Fulcher–Tammann fits were performed on the α-relaxation. Our important contribution for the bi-axially stretched PEN films study is related to the assignment of the β *-relaxation that can be attributed to naphthalene aggregates.

Dielectric relaxation of liquid crystalline dendrimers and liquid crystalline polymers with pendant nitro groups

The dielectric relaxation was determined in a time-domain spectrometer between 100 and 295 K and from 0.1 to 1000 Hz for series of dendritic liquid crystalline polymers with scaffolds based on 2,2-bis(hydroxymethyl)propionic acid ( bis-MPA) of generations 0–3 with 4″-(( R)-1-methylheptyloxy)phenyl 4-{4′-[10-(hydroxy-carbonyl)decyloxy]-phenyl} benzoate as mesogen and for the second generation of dendrimers with 4″-[10-(hydroxycarbonyl)decyloxy]-biphenyl 4-[4′-(2-( R)-octyloxy)-3′-nitrophenyl] benzoate as mesogen. Dielectric data are also reported for a side-chain liquid crystalline polymer based on {4″-[( R)-(−)-2-octyloxy]-3″-nitro-phenyl 4-(4′-(11-acryloyloxyundecyloxy)phenyl}benzoate. The low temperature subglass process (γ) assigned to the spacer group was essentially insensitive to molecular architecture and to the presence of a pendant nitro group in the mesogen. The high temperature sub-glass process (β) assigned to a 180° flip of part of mesogen was only moderately sensitive to the molecular architecture. The introduction of a pendant nitro group in the mesogen led to a substantial increase in both activation energy and relaxation strength. The results suggest that the phenylene ring with the pendant nitro group participates in the β process.

Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate). The strength of the dielectric glass-rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical a peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called γ relaxation, centered in the vicinity of -50 °C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass-rubber relaxation. The mean-square dipole moments per repeating unit, /x, measured at 25 °C in benzene solutions, are 2.5 D 2 , 1.9 D 2 , and 5.0 D 2 for poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross-correlation contributions to the dipolar correlation coefficient may have the same time-dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass-transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 °C and 32 °C above the T g 's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers.

Chain dynamics in the nematic melt of an aromatic liquid crystalline copolyester: A molecular dynamics simulation study

A molecular dynamics (MD) simulation study of chain dynamics and the relation to dipolar relaxation has been carried out on a 70/30 composition random copolymer of p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid. The dynamics is analyzed in terms of the relatively flexible torsion at the ester oxygen-aromatic carbon bond. The effective torsional potential in the bulk that results from sampled torsional angle populations is found to differ from that of free chains in that the barriers are higher in bulk and the torsion connecting phenyl and naphthyl units has a higher barrier than that between pairs of phenyl units. Activation energies for conformational transitions are higher than those in the effective potentials. These effects indicate sensitivity to packing not found in simpler systems like polyethylene, and that frictional effects are operative. The distribution of transitions over individual bonds is found to be quite heterogeneous over the temperature range studied and is a signature for a vitrifying system. Conformational jumps were not found to be highly correlated, although some preference for next-neighbor events was indicated. Dipolar time autocorrelation functions were constructed separately for phenyl and naphthyl dipoles. The naphthyl units were found to relax more slowly than the phenyls. Comparison of MD relaxation times with experimental loss maxima indicates a degree of correspondence that lends credence to the assignment of the experimental β relaxation subglass process as associated with the naphthyl units and the γ subglass process with the phenyl units.

Molecular Packing and Dynamics of the Main Chain and Side Chains in Mesomorphic Poly(vinyl ether)s As Revealed by X-ray Scattering, Dielectric Spectroscopy, Solid State 2H, and 13C-MAS NMR Spectroscopy

The conformational states and dynamics of a series of side-chain liquid crystalline poly(vinyl ether)s with phenylbenzoate or stilbenzyloxy as mesogen and with 4−11 methylene spacer groups, where the 11-methylene-spacer polymers had two different tacticities, were assessed by dielectric and NMR spectroscopy. Dielectric spectroscopy confirmed the presence of three subglass processes (β, γ, and δ) exhibiting an Arrhenius temperature dependence. The activation energy (35 ± 5 kJ/mol) of the γ process was insensitive to the morphology, and this process is assigned to localized motions within the spacer group. The absence of the β process in the stilbenzyloxy polymer suggests that this process is due to flips of the phenylene units causing a 180° reorientation of the carboxylic group in the phenyl benzoate moiety. The frequency of the flip of the inner phenylene unit at 0 °C as obtained by 13C-MAS NMR was almost the same as the frequency of the dielectric β loss peak at the same temperature. The relaxation strength of the β process passed through a maximum as the spacer length increased. It is suggested that the participating carboxyl groups were unable to reorient completely in a polymer with a short spacer group since reorientation of the mesogen was more complete in a polymer with longer spacer groups, the latter containing a smaller percentage of mesogens. 13C-MAS NMR data indicated that the gauche content of the spacer group was lower than that of a free polymethylene chain at the same temperature and that it increased with increasing temperature. The X-ray scattering patterns of the 11-methylene-spacer polymers with different tacticities were identical, and it was shown by NMR and dielectric spectroscopy that the molecular dynamics was not influenced by the tacticity of the polymers. The decrease in the thickness of the smectic layer at the transition from smectic B to smectic A is accompanied by a pronounced increase in segmental mobility of the backbone as revealed by 2H NMR spectroscopy.

High Subglass Transitions Appearing in the Rigid Polyimides Derived from the Novel 1,6-Bis(4-aminophenyl)diamantane

This work has synthesized new diamantane-based polyimides by reacting 1,6-bis(4-aminophenyl)diamantane (5) with various aromatic tetracarboxylic dianhydrides. Dynamic mechanical analysis (DMA) reveals that diamantane-based polyimides have three relaxations on the temperature scale between 0 and 500 °C. The low-temperature subglass relaxations, ranging from 150 to 200 °C, are typical β relaxations for standard polyimides. The relatively high-temperature β1 subglass relaxations in polyimides 8 occur at a markedly higher temperature, i.e., approximately 300 °C, than typically observed in most other polyimides. The characteristic β1 relaxation is associated with a step decrease in G‘ and small transition peaks appearing in tan δ and G‘‘. Moreover, the insensitivity of the β1 subglass relaxation temperature to changes in the dianhydride demonstrates that the subglass process is localized largely within the diamine moiety. Their glass relaxations occur at extremely high temperatures exceeding 500 °C. These films had low dielectric constants ranging from 2.54 to 2.74, as well as low moisture absorptions less than 0.40%. In addition, the polyimides 8d and 8e are soluble in o-chlorophenol, chloroform, N-dimethylacetamide (DMAc), and tetrahydrofuran (THF). These films have tensile strengths to break values up to 124 MPa, elongation to break values up to 5.6%, and initial moduli up to 2.3 GPa. Interestingly, selective functionalization of 1,6-dibromodiamantane to 1,6- and 4,9-diphenyldiamantanes is attained using the catalysts FeCl3 and AlBr3, respectively.

Relaxation processes in hyperbranched polyesters: influence of terminal groups

Three hyperbranched polyesters with the same backbone structure but with different terminal groups: hydroxyl, benzoate or acetate groups, were studied by dielectric spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. The benzoate- and acetate-terminated polymers exhibited only one subglass process (β), originating from reorientation of the ester groups, distinct from the glass transition (α). The hydroxyl-terminated sample exhibited a low-temperature subglass process (γ), due to motions of the hydroxyl groups, in addition to the β and α processes. The relaxation strengths of the hyperbranched polymers were found to be considerably lower than for those of linear analogues. The activation energies of the β process in the polymers studied increased in the order of hydroxyl, acetate and benzoate, indicating that the benzoate-terminated polymer is the most constrained.

Dielectric relaxation of liquid crystalline polymers used in non-linear optical materials

Dielectric permittivity and loss have been measured over the frequency range 0.1–1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of smectic monomers. The polyacrylates were based on mixtures of chiral monofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and nonchiral bifunctional monomers, either with an ester group in the mesogen or with an ester group in the mesogen and a lateral nitro group, creating networks with different crosslink densities. Poled copolymers based on these monomers exhibit second-order nonlinear optical properties. Polymers based only on monomers with the chiral and nitro groups exhibited a pronounced γ process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temperature subglass process (β) associated with reorientation of the mesogen. The polymers with the bifunctional monomer without the nitro group exhibited clearly both γ and β processes. The dielectric data indicate that the ester groups in the bifunctional units undergo conventional reorientation according to the β mechanism and that the β process is suppressed in the pendant chains with the lateral nitro groups. The β process involves a coordinated torsion about two bonds in the mesogen and the swept-out volume involved in the motion becomes extensive when a lateral nitro group is present. © 1997 Elsevier Science Ltd.

Dynamics of Liquid Crystalline Side-Chain Poly(Vinyl Ether)s

Abstract Segmental mobility between 100 and 350 K is reported for a series of mesomorphic side-chain poly(vinyl ether)s with 4–11 methylene carbon atoms in the spacer and phenyl benzoate and stilbenyloxy groups used as mesogens. A glass-rubber transition (α) appears in the vicinity of 300 K. Three subglass processes, referred to as β, 7, and δ obey the Arrhenius and Cole-Cole equations. The δ-process is present only in polymers with flexible and dielectrically active tail groups, e.g., in polymers with an ethoxy group, in which case the process can be assigned to torsion about the pendant phenyl-carbon-ether-oxygen bond. The activation energy (35 ± 5 kJ ·−1) of the β-process is insensitive to the morphology and is assigned to local motions within the spacer. The activation energy of the β-process varied considerably among the studied polymers (60 to 126 kJ·mol−1). The absence of the β-process in the stilbenyloxy polymer suggests that this process is due to reorientation of the carboxylic group in the phen...

Relaxation processes in hyperbranched polyesters

Dielectric permittivity and loss have been measured over the frequency range 10-2 Hz-10 kHz between 100 K and 350 K for samples of 50/50 mixtures of each of two hyperbranched polyesters, one five-generation hydroxy functional (5G-OH) and one threegeneration alkyl-terminated polymer (3G), with dielectrically inactive linear polyethylene. The thermal transitions of the hyperbranched polymers were studied with differential scanning calorimetry. Three relaxation transitions were found in 5G-OH: α, the glass-rubber transition and two subglass processes denoted β and γ showing Arrhenius temperature dependence both with an activation energy of 96±2kJ mol-1. The low temperature process could be assigned to motions of the terminal hydroxyl groups whereas β is due to reorientation of the ester groups. Sample 3G showed only a glass transition and one subglass process being assigned to reorientation of the ester groups. The high activation energy (202 kJ mol-1) of this process indicates that the ester groups are highly constrained in this polymer.

Dielectric relaxation of liquid crystalline side-chain poly(vinyl ether)s

Dielectric permittivity and loss have been measured over the frequency range 10−2 Hz–10 kHz between 100 K and 350 K for samples of two mesomorphic side-chain poly(vinyl ether)s, poly(4-(11-(vinyloxy)undecyloxy)-4-ethoxyphenylbenzoate) (P-EtO) and poly(4-(11-(vinyloxy)undecyloxy)-4-cyanophenylbenzoate) (P-CN) of different degree of mesogen group orientation. X-ray scattering showed that P-EtO was in a semicrystalline state below 345 K, whereas P-CN displayed a titled smectic structure at temperatures lower than 315 K. Four relaxation transitions were found in P-EtO: α, the glass-rubber transition occurring at 290–300 K, and three subglass processes referred to as β, γ and δ. P-CN exhibited only three dielectric processes, α, β and γ. The low temperature process, δ, was absent in P-CN and could be assigned to torsion about the pendent phenyl-carbon-ether-oxygen bond. Subglass processes β and γ exhibited an Arrhenius temperature dependence with activation energies of 60–105 and 32 ± 2 kJ mol−1, respectively. The activation energy of the γ process was insensitive to morphology and it was assigned to local motions in the spacer group. The activation energy of the β process varied considerably among the studied polymers with higher values for the highly ordered P-EtO than for P-CN. It is suggested that the β process leads to reorientation of the carboxylic group in the phenyl benzoate moiety.

Evidence of fast diffusive process in a mixed polymeric glass

the molec­ular motion in polymers is restricted to specific groups or structural rearrangements responsible for secondary relax­ations. Numerous studies of subglass processes have been reported by utilizing mainly dynamic mechanical (MS) and dielectric (DS) spectroscopic techniques. There is a consensus about the Arrhenius temperature dependence of the characteristic relaxation rates, the inherent broad dis­tribution, as well as the short correlation length of the underlying thermal fluctuations. Therefore, the observed agreement between MS, DS, and light scattering results for one polymeric glass system