Transition Of Polystyrene Research Articles

Room temperature brittle-to-ductile transition in polystyrene induced by extrusion casting melt stretching: Contribution of free volume and chain entanglement

In this paper, a series of polystyrene films with different melt stretching strains were prepared using the extrusion casting melt stretching method. At room temperature, the elongation at break of polystyrene can reach up to 70 %, showing a brittle-ductile transition behavior below the glass transition temperature. In order to clarify the structural and performance changes caused by melt stretching, the hardening modulus and entanglement density were calculated using the rubber elasticity theory, and the free volume parameters were calculated using Positron Annihilation Lifetime Spectroscopy (PALS). It was found that melt stretching resulted in an increase in the hardening modulus and entanglement density, with minimal change in free volume size but a significant increase in free volume concentration. The rise in free volume concentration led to a decrease in yield strength, elastic modulus, and the yield energy barrier. Concurrently, the brittle-to-ductile transition temperature exhibited a linear decrease with increasing free volume concentration. When the brittle-to-ductile transition temperature approached room temperature, the sample displayed a physical aging resistance of over 9 months. The increase in chain entanglement density facilitated the stable formation of a craze zone during the tensile hardening stage, enabling ductile deformation of the polystyrene film. By tracking the structural evolution of the sample with a melt stretching strain (MSS) of 4.23 during the aging stage under room temperature, we found that the aging process is a process in which the free volume concentration decreases, resulting in an increase in the yield strength and an increase in the yield energy barrier. Chain entanglements formed by melt stretching relax and disentangle during the aging stage, but interchain interactions form cohesional entanglements. As secondary entanglement, cohesional entanglement undergo slip failure when stretched and cannot promote the formation of craze areas. Hence, free volume concentration and chain entanglement density emerged as two pivotal factors influencing the brittle-ductile transition of polystyrene below the glass transition temperature. The melt stretching process increased the free volume concentration and chain entanglement density in polystyrene, achieving a brittle-to-ductile transition below the glass transition temperature.

Mechanism and quantitative study of specific heat change during glass transition of amorphous polystyrene and Pd<sub>40</sub>Ni<sub>10</sub>Cu<sub>30</sub>P<sub>20</sub>

<sec>The nature of glass transition is one of the most interesting problems in modern condensed matter physics. There is a theory that shows that when the particle's diffusion motion probability <i>P</i> is less than <inline-formula><tex-math id="M2">\begin{document}$ {{\rm{e}}^{ - 2{{\rm{e}}^3}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200331_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200331_M2.png"/></alternatives></inline-formula>, the proportion of particles whose diffusion motion is frozen is not less than 1 + 2e<sup>3</sup>/ln<i>P</i>. Based on the modulus temperature formula of this theory and the experimental modulus-temperature curve of polystyrene in the literature, the free volume expansion coefficient of polystyrene is determined to be in a range of 0.00045－0.00052. </sec><sec>In this paper, we first quantitatively study the specific heat temperature relationship in the glass transition of polystyrene. Based on this theory, the specific heat-temperature formula in the glass transition region is derived. The material in the glass transition region is a mixture of rubber and glass, and this two-component (rubber and glass) system’s total specific heat is the product of the specific heat of the rubber and the percentage of rubber in the two-component system plus the product of the specific heat of the glass and the percentage of glass in the two-component system. Let <i>b</i> be the number of atoms in the main chain of the segment, then the proportion of rubber will be the <i>b</i>-th power of free volume fraction. This specific heat-temperature formula with polystyrene is tested. By substituting the obtained free volume expansion coefficient of the polystyrene into the specific heat-temperature formula, the resulting formula can accurately and quantitatively describe the specific heat-temperature relationship in the glass transition of polystyrene without fitting any parameters. </sec><sec>In this paper, we also study the change of motion in the glass transition of polystyrene. According to the analysis, the glass transition process of polystyrene is a process in which the diffusion movement of the main chain atoms is activated or frozen, which is consistent with the conclusions of relevant research on amorphous alloys. When the main chain atom is used as the molar unit of measurement, the specific heat change in the glass transition of polystyrene is 1.61<i>R</i> (<i>R</i> is the gas constant), which is consistent with the law, i.e. “the specific heat change in the glass transition of the amorphous alloy is about 1.5<i>R</i>”. These consistent conclusions predict that the glass transitions of amorphous alloys and glass transitions of polystyrene have the same essence. Based on this idea, the specific heat temperature formulas of the amorphous alloy Pd<sub>40</sub>Ni<sub>10</sub>Cu<sub>30</sub>P<sub>20</sub> and polystyrene are verified and prove to be consistent.</sec>

Conformational transition of polystyrene in good solvents studied by fluorescence spectroscopy

In this paper, the conformational transition of polystyrene (PS) in good solvents (i.e., dichloroethane and decalin) is studied by intramolecular fluorescence resonance energy transfer (FRET, NRET). The characteristics of FRET and PS excimer fluorescence spectroscopy in the analysis of polymer solution are systematically described. We find that FRET is prior to PS excimer fluorescence spectroscopy due to the superior sensitivity and better spectroscopic signal resolution. Based on the FRET results, it can be concluded that the chain conformation almost keeps unchanged with the increase of polymer concentration before the concentration reaches the critical overlap concentration ( C *). This spectroscopic observation excludes the existence of dynamic contact transition. In the expected C * regime, a sharp break point is observed for the fluorescence intensity versus the increase of the concentration, which is in exactly agreement with the theoretical value of C *. Additionally, the polymer chain undergoes severe collapse when the concentration reaches the semidilute regime and increases continuously. This work demonstrates that intramolecular FRET is a robust technique to sensitively characterize C *. C * could be used as a threshold to describe the conformational transition of polymer in good solvents.

Strain rate-regulated sub-Rouse transition in polystyrene via dynamic mechanical spectroscopy

The sub-Rouse mode in polystyrene after deformation was investigated via dynamic mechanical spectroscopy (DMA). Different tensile rates were adopted during the deformation, and the influence of tensile rate on chain relaxation behaviors and mechanical properties were analyzed thoroughly. The spectrum of internal friction is consisted by three relaxation peaks, local segmental relaxation (α transition), sub-Rouse mode (α′ transition), and Rouse mode (α″ transition), respectively. Significantly, the sub-Rouse relaxation intensity increases with the growing of the tensile rate at the same degree of deformation, and the tensile rate of 0.5 mm/min is regarded as the turning point to sub-Rouse relaxation. According to the strain-stress curves and the phenomenon of sliver crazes, it was inferred that some microcosmic orientation formed in the process of the deformation and the orientation structure is considered as the seed of the sliver crazes. What is more, the variation of sub-Rouse relaxation could also be attributed to the generation of the orientation.

Dual beam differential photopyroelectric setup for broadband thermal effusivity investigation of glass transitions in polymers

The behaviour of the thermal effusivity squared over the glass transition in polystyrene is measured over a considerable frequency range using a novel dual beam front detection photopyroelectric configuration, which ensures a very effective processing of the obtained signal. This has enabled the detection of the very small changes taking place in such a parameter, despite the non-favourable sensitivity conditions due to the large mismatch between the thermal effusivities of the pyroelectric transducer and of the sample. The effect of the thermal contact resistance between the sample and the pyroelectric detector is also considered and discussed.

Pressure Jump Kinetics of Disorder to BCC Ordering in Diblock Copolymer Micelles in a Selective Solvent

We present time-resolved small-angle X-ray scattering (SAXS) measurements of the kinetics of a barotropic disorder–order transition in polystyrene–polyisoprene (SI) diblock micelles in diethyl phthalate (DEP), a styrene selective solvent. Because of the fast equilibration in a pressure jump we were able to observe the initial induction stage during which the short-range order of the micellar fluid increases, followed by a sharp first-order nucleation and growth of the BCC phase and eventual late stage coarsening. The time evolution of intensity from individual diffraction spots of a crystallite was obtained from the analysis of 2-dimensional SAXS patterns and shows late stage power-law growth. The time evolution of the SAXS intensity profiles was analyzed by fitting the contribution to the scattering from the ordered Bragg peaks by Gaussians and analyzing the contribution to the scattering from the disordered micellar liquid in terms of the Percus–Yevick interacting hard-sphere model. The time evolution of the micellar structural parameters obtained from this analysis reveals that the disorder to order transition occurs when the volume fraction of micelles reaches 0.53, and the effective hard sphere radius and volume fraction of micelles increases upon ordering while the core radius is unaffected. In both the ordered and disordered states the hard sphere radius and volume fraction of micelles increase with increasing pressure indicating increased swelling of the micellar corona. In the ordered state there is considerable penetration of the corona chains from neighboring micelles.

Study on O-Terphenyl/Polystyrene Blends: How Plasticizer Affects the Glass Transition of Polystyrene?

The effect of plasticizer o-terphenyl on the glass transition of polystyrene was investigated by Fourier transform infrared spectroscopy from a new aspect. The peak areas of four conformation insensitive bands as a function of temperature were studied, these being assigned to the vibrational modes of main chain groups and side groups of polystyrene. It was shown that the reorientation relaxation temperature of the main chain around glass transition was lower than that of the side groups when polystyrene was plasticized by o-terphenyl. It was explained on the basis of cohesional entanglements of polystyrene chains. The reorientation relaxation region of the side groups was nearly the same as the macroscopically observable glass transition region of polystyrene, implying that the glass transition process of polystyrene was dominated by the reorientation of side groups.

New Insights into the Effects of Physical Aging on Glass Transition of Polystyrene by FTIR

A new method to detect the glass transition was used to investigate the effects of physical aging on the glass transition of polystyrene by Fourier transform infrared (FTIR) spectroscopy. The results showed that the onset point of the glass transition shifted to higher temperature with aging, but the end point didn't change. It was demonstrated that this method was of great use in determining the glass transition temperature of aged polymeric samples. For each aged sample, the four bands assigned to the vibrational modes of the main chain groups and the side groups of polystyrene showed the same transition regions, indicating that the main chain and the side groups were aging synchronously. The rates of increase of the relative peak areas for the four investigated bands during the glass transition decreased with aging with the increased rates of the relative peak areas for the two bands assigned to side group decreasing less than those of main chain, which seemed to indicate the dominance of the side group in aging process.

Effects of molecular weight on the liquid-liquid transition in polystyrene melts studied by low-frequency anelastic spectroscopy

A substantial internal friction peak associated with the liquid-liquid transition (Tll) has been observed in polystyrene (PS) melts with different molecular weights Mw. The peak is of the relaxation type and suggested to be caused by the cooperative rearrangement of PS chains. The relaxation time follows the Vogel–Fulcher–Tamman equation. With increasing Mw, the PS melt exhibits a higher energy barrier, a smaller concentration of mobile species, and a stronger coupling between mobile species at Tll. In addition, to quantify the strength of the temperature dependence of the relaxation time, a parameter mll is defined for PS melt according to Angell’s fragility concept. The value of mll decreases with increasing Mw, indicating a slower cooperative rearrangement of PS chains toward Tll. Moreover, at Mw⩽52.5kg∕mol, mll rapidly drops with Mw, while it more slowly decreases at Mw>52.5kg∕mol. The fact suggests more topological constraints due to the intrachain interactions in very long chains.

Dynamic relaxation of polystyrene/poly(ethylene oxide) blends above glass transition temperature

A series of blends with different compositions were prepared by melt-mixing on the basis of polystyrene and poly(ethylene oxide). Using the relative energy dissipation method, the dynamic behavior of the blends above the glass transition temperature was investigated. Two relaxational dissipation peaks (α and α′) were observed. It is suggested that peak α is associated with the glass transition of polystyrene and peak α′ with the liquid-liquid transition. The relaxation time τ of both relaxations does not satisfy the Arrhenius law. The effect of composition on the dynamic relaxation of the blends were also discussed qualitatively.

Pressure induced order–disorder transition in a diblock copolymer

Polymers are known to undergo order↔order and order↔disorder transitions, when subjected to a change of pressure, temperature, solvent, pH of the medium etc. The molecular processes, which alter the volume of the system, are found to be highly sensitive to the pressure. In the present communication Zimm and Bragg model of helix↔coil transition has been modified to interpret the experimental data of pressure induced phase transition in polystyrene–polybutadiene [PS–PB] at different pressurization rates, as reported by Migler and Han, utilizing the Birefringence and small angle neutron scattering technique. An expression for the degree of order is obtained from the grand partition function for the entire chain in terms of nucleation parameter, which controls the transition width. The nucleation parameter σ increases with the increase in the ODT temperatures of the system. The phenomenon of hysteresis has been discussed in relation to the pressurization rate, which increases/decreases with the corresponding increase/decrease in the pressurization rate. The theoretical transition curves are found to be in good agreement with experimental data.

Simultaneous measurement of thermal expansion and heat capacity using temperature modulation technique

An instrument for simultaneous measurement of thermal expansion and heat capacity was developed using temperature modulation technique. The instrument was designed for thin films in order to avoid temperature distribution in the sample. Electric capacitance was measured to obtain temperature dependence of the sample thickness. The instrument was applied to the glass transition of polystyrene and it was confirmed that reliable comparison between the dilatometric and calorimetric results could be made.

Variation of the Effective Activation Energy Throughout the Glass Transition

AbstractSummary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B2O3). The values of E decrease from 280 to 120 kJ · mol−1 in PS, from 1 270 to 550 kJ mol−1 in PET, and from 290 to 200 kJ mol−1 in B2O3. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers.Variation in the effective activation energy of PS, PET, and B2O3 with temperature.imageVariation in the effective activation energy of PS, PET, and B2O3 with temperature.

Study of the conformational transition of polystyrene?poly(methyl methacrylate) block copolymers with temperature using measurements of the c** critical concentration

The conformational transition of block copolymers has been studied until now using measurements of the second virial coefficient and the radius of gyration (light scattering) or of the intrinsic viscosity. In this paper the conformational transition of polystyrene–poly(methyl methacrylate) diblock copolymers with temperature is studied using measurements of the critical concentration c** which is obtained in the dynamic state (viscometry) and in the static state (UV measurements).

The study of polystyrene-fullerene solid-phase composites

Polystyrene-based films containing from 0 to 45 mol % of C60 fullerene are investigated. It is found that the introduction of fullerene increases the molecular packing density of polystyrene chains, thus affecting the transport of small molecules through the polymer films. Gas diffusion through the composite films is slower than through the polystyrene films, while the gas-distributing properties of the composites are higher. The dielectric relaxation method shows that the solution-grown films retain the cluster state of fullerene. When the films containing more than 0.15 mol % of fullerene are heated above the glass transition temperature of polystyrene away from air, the relaxation time of the α transition in polystyrene is found to increase. This effect is associated with strong chain interaction via fullerene molecules incorporated into polystyrene-C60 complexes. The NMR spectroscopy method allows us to observe the typical changes in the composites.

Chemical and vibronic effects in the high-resolution near-edge X-ray absorption fine structure spectra of polystyrene isotopomers

This Letter presents the high-resolution C 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of hydrogenated and deuterated polystyrene. The differences between these spectra provide unambiguous evidence for the presence of a significant vibronic contribution to the shape and structure of the C 1s(C–H) →1π ∗ C C transition in polystyrene. High-resolution NEXAFS spectra, spectroscopic simulations and ab initio calculations are used to help resolve the relative contributions of chemical shifts and vibronic excitation to the shape of the characteristic C 1s(C–H) →1π ∗ C C transition.

Transitiometric Analysis of Pressure Effects on Various Phase Transitions

A scanning transitiometer has been used in investigations of 1st and 2nd order phase transformations in polymers. It was demonstrated taking as an example fusion of polyethylene at 200 MPa with a temperature scan as inducing variable that by recording simultaneously the rate of heat exchange and the rate of volume variations it is possible to determine in a single experiment the pressure derivative of temperature of this 1st order phase transition. For phase transformations similar to the 2nd order transitions the transitiometric analysis permits simultaneous measurements of pairs of thermodynamic derivatives which permit determination of pressure effects according to the Ehrenfest equations. For the glass transition in polystyrene at high pressures the pressure effect was similar independently of the pair of thermodynamic derivatives used (heat capacity and thermal expansivity or compressibility and thermal expansibility).

The conformational coil‐globule transition of polystyrene in cyclohexane solution

AbstractThe phase diagram of a monodisperse high molecular weight polystyrene (Mw = 3.3 · 106, Mw/Mn < 1.06) solution in cyclohexane was thoroughly studied by means of light scattering, cloud point, coexisting phases volume ratio, coexisting phases composition and scanning calorimetry methods, and binodal and spinodal curves were obtained. Coordinates of the UCST (T = 306.2 K, ϕ2 = 1.88 · 10−2) determined by different methods were similar. It was found that the first‐order phase transition curve obtained by calorimetric scanning at 10−6 K/s accurately fits the spinodal of the system within the concentration range of semidilute and concentrated solutions and has a horizontal part in dilute solutions where the temperature of phase transition was found to be 303.86 K, independently of concentration. At the same temperature a deflection point was found on the concentrated branch of the spinodal. The results were taken as calorimetric evidence of a coil‐globule phase transition in the polystyrene solution. It was shown that the collapse of the polystyrene coils takes place in the metastable system between the binodal and the spinodal in the same way as in the homogeneous solution above the binodal.

Order-disorder transition of polystyrene-block-polyisoprene. I. Thermal concentration fluctuations in single-phase melts and solutions and determination of χ as a function of molecular weight and composition

Small-angle x-ray scattering (SAXS) from single-phase melts and solutions of polystyrene-block-polyisoprene (SI) having various degrees of polymerization N and compositions f were analyzed on the basis of Landau-type mean-field theory developed by Leibler. The effective Flory interaction parameter χeff per segment in melts and in semidilute Θ solutions were determined from the mean-field analysis of the SAXS profiles obtained at temperatures higher than TMF, the crossover temperature above which the non-mean-field effects become less significant and the mean-field theory becomes a good approximation. It was found that χeff thus determined depends on N, f, volume fraction of the SI in the solution φp and the interaction parameter χ0 in melt: χeff=φpχ0 and χ0 is a function of f and N; χ0 at a given f is a decreasing function of N and that at a given N is approximately a quadratic function of f with a minimum at f≂1/2.

Boson peak and fast relaxation process near the glass transition in polystyrene

Atactic polystyrene, both side group and main chain deuterated, was investigated by inelastic neutron scattering in a wide temperature range around the glass transition from 2 to 450 K. In the glass the Boson peak position is only very weakly influenced by the deuteration of the phenyl group. In the neighborhood of the glass transition temperatureTg we find a fast relaxation process similar to other glasses. The onset of the fast relaxation in polystyrene, however, is observed already at temperaturesTg — 200 K. Results from partially deuterated polystyrene suggest a change of the phenyl ring dynamics already far belowTg.