Molecular motions in poly(dimethyl siloxane) oligomers and polymers

The glass transition temperatures of fifteen samples of poly(dimethyl siloxane) polymers and oligomers have been measured by differential scanning calorimetry. Polymers with M_n>2400 exhibit an asymptotic value of T_g(∞)=148K, but for shorter chains the T_g is lower. The thermomechanical behaviour has also been examined using a torsional braid analyser and this has served to confirm the chain length dependence of T_g. A sub-glass transition, located in the temperature range 80–120K for short chain samples, has been attributed to methyl group rotation.     

Evaluation of local polarity of polymer solids by a rigid fluorescent probe of carbazole-terephthalate cyclophane

A novel rigid fluorescent probe, carbazole-terephthalate cyclophane (Cz-TP) was applied to evaluate local dielectric constants (ε) of various polymer solids in a wide range of temperatures. For poly(vinylidene fluoride), the ε increased above the glass transition temperature (T_g), due to relaxations of the polar segment -(CH₂CF₂)- of the main chain. For poly(alkyl methacrylate)s, the ε increased above the T_g or the melting temperature of the side chain, where motions of the polar ester groups are activated. For cyanoethylated polymers, the ε increased owing to motions of the polar cyano groups at the end of the side chain and the ε corresponded to the dielectric constant evaluated by dielectric relaxation measurement at a high frequency, because the Cz-TP exciplex has a lifetime of tens of nanoseconds. For a cyanoethylated polymer with a high content of cyano groups, the ε was larger at low temperatures than the dielectric constant obtained by the macroscopic dielectric relaxation measurement. These results show that the Cz-TP molecule is a useful probe for evaluation of the local polarity in polymer solids over a wide temperature range and can detect even a small change in ε at transition temperatures such as glass transition, side-chain melting, and side-chain relaxation.     

Polymerization of linear aliphatic diamine-based benzoxazine resins under inert and oxidative environments

A series of linear aliphatic diamine-based benzoxazine monomers have been studied. Reaction times and purification procedures have been optimized for each individual diamine. The structure of these diamine-based benzoxazine monomers has been characterized by ¹H and ¹³C NMR, and infrared spectroscopy. The rate of polymerization has been studied by Fourier transform infrared spectroscopy as a function of the chain length of the aliphatic amines. The glass transition temperatures (T_g) of the polybenzoxazines from these monomers are also studied. The short chain amine polybenzoxazine exhibits the T_g of around 170 °C. The influence of the polymerization environment for these linear aliphatic diamine-based series of benzoxazine monomers has been studied under air and inert atmosphere. Differential scanning calorimetry is used to determine the melting points of these benzoxazines and the temperature of the peak polymerization exotherm. An anomalous polymerization behavior of ethylene diamine-based polybenzoxazine is also reported.     

Energetic analysis of the two PMMA chain tacticities and PMA through molecular dynamics simulations

Simulated dilatometry techniques have been applied to compute the glass transition temperatures, T_gs, of the two poly(methyl methacrylate) (PMMA) chain tacticities, and poly(methyl acrylate) (PMA). Since the difference in T_gs between the two configurations was accurately simulated, further analysis could be carried out. This article more particularly deals with energetic and structural analysis of the difference. Thus this analysis showed that the non-bond energy and the bending angle energy associated with the intradiad backbone angle, principally contribute to the energetic difference between the two PMMA configurations. Following the free volume theory, these two energetic variations allow an increase in T_g in comparison to PMA, and an enlargement of the difference in the T_gs between the two PMMA configurations. Actually, these two energetic contributions stem from the substitution of the hydrogen atom attached to the chiral carbon atom in the PMA repeat unit by a methyl group. The same behavior is encountered with the two poly(ethyl methacrylate) (PEMA) chain tacticities.     

Effects of molecular weight and chain ends on glass transition of polystyrene

The main glass transition temperatures, T_g of narrow distribution anionic polystyrene samples have been measured by differential thermal analysis (d.t.a.). The d.t.a. data were verified by differential scanning calorimetry. Effects of heating rates were eliminated by extrapolating observed T_g values to a standard heating mode of 1°C/min In a T_g vs. log (heating rate) relation. The variations of T_g peak height and temperature with heating rate and polymer molecular weight (M) agree with conclusions from other reports of d.t.a. work. The T_g values corrected for heating rate effects agree well with figures reported in studies with d.t.a. and other techniques for locating vitrification temperatures.The volumes attributable to end groups differ significantly between anionic and thermally initiated polystyrenes at temperatures above T_g. A comparison of T_g values of both polystyrene types provides a direct test of the iso-free volume model of the vitrification process. A common T_g vs. M relation was found for anionic and thermally initiated polystyrenes, regardless of chain end nature. This comparison is not consistent with the concept that vitrification is governed by free volume defined as the difference between specific volume of the polymer and a reference volume. Our results are compatible with several other mechanisms which have been suggested for the glass transition.     

The adhesion of amorphous polystyrene surfaces below Tg

The bonding of polystyrene (PS) surfaces below T_g was investigated by two different fracture tests: the lap-shear joint method and the cantilever beam method. Adhesion energy values obtained by the two methods are in agreement and develop with (time)^1/2, at temperatures as low as T_g−16 °C. Even if the double cantilever method is the most common test found in the literature for adhesions above T_g, for low adhesion values, below T_g, the lap-shear joint geometry is more appropriate. Moreover, when the glass transition temperature is used as a reference temperature, polydisperse and monodisperse PS adhesion energy curves are superposable, suggesting that the auto-adhesion is not significantly favored by the presence of numerous chain ends at the surface (due to the low molecular weight chains provided by the polydisperse PS).     

Glass transition relations in ionomeric “comb” polymers

The glass transition temperatures (T_g) and dynamic mechanical properties of random copolymers of styrene-4-vinylpyridine quaternized with iodoalkanes are presented for vinylpyridine contents up to ten percent and pendant alkyl chains up to ten carbons in length. Two linear relationships, between ion content and glass transition temperature and between pendant alkyl chain length and the T_g, are observed. An equation is presented that can be used to predict the T_g for such copolymers based on their ion contents and alkyl chain lengths.     

Calorimetric study of some poly(mono-n-alkyl itaconates)

A calorimetric study of poly(mono-n-alkyl itaconates) containing 8, 10, 12 and 14 carbon atoms in the side chain was performed. No glass transition temperature (T_g) was observed in the temperature range 213-263 K, but a melting process was observed for the dodecyl and tetradecyl derivatives, which was attributed to crystalline order in the long side chains.     

Strain recovery of post-yield compressed semicrystalline poly(butylene terephthalate)

Cubic specimens of a semicrystalline poly(butylene terephthalate) (PBT) have been compressed up to post-yield deformation levels with a fast (3.0 × 10⁻² s⁻¹) and a slow (1.5 × 10⁻⁴ s⁻¹) strain rate at three different temperatures (25 °C, 45 °C, and 100 °C, i.e. below, close and above the glass transition temperature of the material, T_g, respectively). Differently from literature results reported for amorphous polymers, semicrystalline PBT shows that, after a post-yield deformation, recovery occurs also at temperatures higher than T_g, and that an irreversible deformation, ?_irr, is set in the material. The irreversible strain component has been evaluated as the residual deformation after a thermal treatment of 1 h at 180 °C.After unloading, isothermal strain recovery has been monitored for time periods of 1 h at various temperatures. From the obtained data, strain recovery master curves have been constructed by a time-temperature superposition scheme. The features of the recovery process for the various deformation conditions have been analysed. In particular, it appears that specimens deformed below T_g show a lower irreversible component, whereas, when deformed above T_g, they display a higher irreversible deformation and a slower recovery process. Moreover, the effect of deformation rate appears particularly marked for samples deformed above T_g.     

Problems of compatibility of the values of glass transition temperatures published in the world literature

The influence of different factors (primarily, the temperature-time conditions for preparation and measurement of samples) on the glass transition temperatures determined from the temperature dependences of properties is analyzed using the calculations performed in terms of the relaxation theory of glass transition. The most optimum conditions for measurement of the glass transition temperatures T _g that ensure the compatibility of the values of T _g obtained by different researchers are recommended. The validity of the assertion that the glass transition temperature T _g is a temperature at which the viscosity of glasses is equal to 10¹³ P is considered.     

Reversible cross-linking reactions of alkoxyamine-appended polymers under bulk conditions for transition between flow and rubber-like states

Reversible cross-linking reactions of alkoxyamine-appended polymers with low glass transition temperature (T_g) were successfully carried out under bulk conditions. The low-T_g polymers with alkoxyamine units in the side chains were synthesised by radical copolymerisation of 2-ethylhexyl acrylate and two kinds of alkoxyamine-containing acrylate monomers. By heating the low-T_g polymers under bulk conditions at 100 °C, cross-linked polymers were formed by radical exchange reactions between alkoxyamine units, and a transition from a liquid-like flowable polymer state to a rubber-like polymer state was confirmed. A de-cross-linking reaction was also accomplished by radical exchange reactions between the cross-linked polymers and an added alkoxyamine-containing small molecule or stable nitroxyl radical, which resulted in transition to the flowable state again. The structural transition between low-T_g linear polymers and cross-linked polymers were characterised by ¹H and ¹³C NMR spectroscopy, Fourier transform infrared spectroscopy, rheology measurement, swelling experiment, and gel permeation chromatography measurement.     

Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures

The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (T_g^PLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (T_c), the glass transition temperature of PS (T_g^PS), the peak melting point of PLLA crystals (T_m^PLLA), and the end melting point of PLLA crystals (T_m,end^PLLA). When annealed at (T_c=) 80 °C (T_c < T_g^PS < T_ODT, order-disorder transition temperature), 123 °C (T_g^PS < T_c < T_m^PLLA < T_ODT), 165 °C (T_g^PS < T_m^PLLA < T_c < T_m,end^PLLA < T_ODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization.     

Influence of alternating sequential fraction on the melting and glass transition temperatures of ethylene-tetrafluoroethylene copolymer

The melting (T_m) and glass transition (T_g) temperatures of a series of ethylene (E)-tetrafluoroethylene (TFE) copolymer (ETFE) have been found to show unique dependence on the TFE content with the minimal and maximal points. These behaviors have been interpreted successfully on the basis of the degree of alternation of E and TFE monomeric units along the skeletal chain. The melting point of a perfectly alternating copolymer is estimated to be 295 °C on the basis of the dependence of T_m using a modified Flory’s equation. The corresponding T_g was estimated as 145 °C by applying a modified Gibbs-Damnation’s equation.     

Fragility and molecular mobility in micro- and nano-layered PC/PMMA films

The values of the fragility index of polycarbonate/poly(methylmetacrylate) multilayer films have been determined for materials having thicknesses varying from micro- to nano-scales. The influence of the layer thickness reduction down to 12 nm has been shown to have very different influence on the glass transition parameters of the two polymers. Polycarbonate has exhibited a huge sensitivity to such geometric scale modifications while no modification has been observed for PMMA. A large decrease of the fragility index m, has been observed for PC. This modification is proportional to length scale variations of molecular cooperative motions at T_g, ξ(T_g). In regard to the molecular structure of each polymer, these results show that this correlation between fragility index and ξ(T_g) is mainly associated to modifications of intermolecular interactions and local density. These material parameters are influencing the activation volume ΔV^# which plays a role on the modification of the fragility index.     

Change of amorphous state of poly(ethylene terephthalate) by heat treatment below the glass transition temperature

Several papers have demonstrated that structural changes in the amorphous regions of semicrystalline polymers can be produced by heat treatment below the glass transition temperature (T_g). In this paper, we report structural change in the amorphous phase of poly(ethylene terephthalate), heat-treated below and above T_g. The density, the T_g, the endothermic peak at T_g and the relative spectral intensity in the 973 cm^?1 band (due to the CO stretching vibration), all increased with heat treatment below T_g, but the specific heat decreased. The stability of the amorphous state was examined by further heat treatment at temperatures above T_g and sufficiently high for crystallization, and it was verified that structural changes in the amorphous regions do not result in acceleration of the rate of crystallization. We therefore suggest that the amorphous region is one phase, rather than two phases consisting of random and regular regions.     

Ultrasonic properties of some diamine cured aliphatic epoxy polymers

Longitudinal sound speed and absorption were measured from 0° to 100°C at a frequency of 2 MHz on an aliphatic epoxy resin (butanediol diglycidyl ether) crosslinked with four different curing agents—three aliphatics of differing chain lengths and one aromatic. The ultrasonic properties were found to be dominated by the glass transition in a manner qualitatively similar to linear polymers. In the aliphatic series, the ultrasonic properties all fell on the same master curves when plotted as functions of T ? T_g, but the aromatic curing agent was qualitatively different. For a single relaxation time model, the sound absorption peak for the aliphatic series would be 55 dB cm^?1. The measured peak of 28 dB cm^?1 shows that the glass transition involves a distribution of relaxation times.     

Effects of temperature on elastic behavior of short compact polymers

In this paper, we study further to explore the effects of temperature on the elastic behavior of short compact polymers. Average conformations and thermodynamics statistical properties at various temperatures T are calculated here. Different chain lengths N and elongation ratio λ are also considered simultaneously. From the plots of f and f_U vs. elongation ratio at low temperature, we can know that compact polymers are more close to the native states. With temperature decreasing deeply, polymer chains have the tendency to form globular structures. The results are concluded from: the ratio of 〈L₁²〉/〈L₂²〉 increase abruptly with temperature decreasing at low temperature, and both characteristic ratio 〈R²〉/Nb² and average energy per bond 〈U〉 decreases abruptly with temperature decreasing at low temperature, here L₁², and L₂² are the eigenvalues of the radius of gyration tensor S (L₁²≤L₂²). We also analyze the relationship between the heat capacity C_V and temperature T for different chain lengths in the process of tensile elongation. The coil-to-globule transition temperature T_c can be estimated from the location of the peak on the heat capacity plot as a function of temperature. The plots of 〈R²〉 as a function of chain length N at different temperatures are also shown, and the correlation 〈R²〉∼N^α is obtained at T>T_c or T<T_c, while at T=T_c, the plots are irregular, here α depends on temperature and elongation ratio simultaneously. Elastic force (f), energy contribution to elastic force (f_U), and the ratio f_U/f are also discussed at various temperatures. These investigations may provide some insights into elastic behaviors of compact polymers at different temperatures, especially at low temperature.     

Influence of molecular weight on dynamic crossover temperature in linear polymers

Dielectric and light scattering spectra of two linear polymers, polyisoprene (PIP) and polystyrene (PS), were analyzed in broad temperature and frequency range above the glass transition temperature, T_g. The crossover temperature, T_C, was estimated using two approaches: (i) derivative analysis of relaxation times proposed by Stickel and (ii) Mode-Coupling Theory approach. Both estimates provide consistent values. T_C varies with molecular weight (MW) in both polymers, while the ratio T_C/T_g changes significantly with MW in PS only. It appears that the segmental relaxation time at T_C has value τ(T_C) ∼ 10⁻⁷ s for both polymers independent of MW and similar to the value reported for many non-polymeric glass-forming systems. No sign of the dynamic crossover has been observed in the chain relaxation around T_C of the segmental dynamics.     

The effect of molecular weight and casting solvent on the miscibility of polystyrene-poly(α-methyl styrene) blends

The glass transition temperatures (T_g) have been measured for blends of polystyrene and poly(α-methyl styrene) in the molecular weight ranges: polystyrene, 2030 < M < 250 000, and poly(α-methyl styrene), 17 000 < M < 250 000. The presence of either one T_g or two has been used as a criterion to determine the miscibility of each blend over a range of compositions, and the T_gs were obtained from measurements of the temperature dependence of the heat capacities of the blends. A sinlge T_g was observed over the entire composition range for blends in which the component molecular weights were both less than 70 000 g mol^?1, when these were cast from toluene solutions. When propylene oxide solutions were used to prepare the blends, this limit dropped to M = 50 000 g mol^?1. By using the appearance of two T_gs as an indication that phase separation had taken place, it was possible to establish regions of miscibility and immiscibility as a function of casting solvent and molecular weight for both components. The change in heat capacity at the glass transition was measured and it was found that for miscible blends the heat capacity changes are similar to the calculated values, but for immiscible systems the measured change is smaller than expected.     

Glass transition temperatures of poly N — (N — alkyl)—maleimides

The glass transition temperatures (T_gs) of a series of poly N-(n-alkyl) maleimides covering only the even members with side chains ranging in length from ethyl to n-octadecyl have been studied from room temperature to above T_g. T_gs and thermal quantities have been determined from the specific volume-temperature relations only for the higher (n=8, 10, 12, 14, 16 and 18, where n = no of CH₂) members of the series. However for the lower ones (n = 2, 4, 6, 8 and 10) T_gs have been detected from heat capacity-temperature traces of differential scanning calorimetry diagrams by extrapolation to zero rate of heating. Accurate consistency was found in the values (n = 8 and 10) determined by both experimental methods. T_gS of these polymers continuously decrease as the number of methylene groups in the side chain is increased, and they have been correlated with the size of the n-alkyl group in the side chain. The results are in accord with a previously studied series concerning the effect of a long side chain on the T_g of a comb-like polymer in the amorphous state. T_gs of poly N-(n-alkyl) maleimides encompassing a wide range of methylene group content (n = 2, 4, 6, 8, 10 and 12) have been examined according to the Gordon-Taylor-Wood extrapolation with the objective of ascertaining the T_g of polyethylene (PE). Our approach of ignoring higher members of the homologous series in this extrapolation appears to be old and well known and it has been variously ascribed to different authors. Extrapolation of T_g values to 100% amorphous PE gives a T_g of 200/pm 10 °K in complete agreement with recent predictions made by Boyer from different sources of data. The Simha-Boyer free volume quantity Δa. T_g decreases slowly with the methylene group content in the longer terms (n = 8, 10, 12, 14, 16 and 18) of the series presumably because of a reduction in the polarity or an in-chain crankshaft loss mechanism which generates free volume in the glassy state, as stated by Boyer. T_gs do not correlate very well with the contributions of the atomic groups to the cohesive energy density (c.e.d.) so it can be concluded that c.e.d. is not the only factor determining T_g. However, a somewhat improved relationship might be obtained by taking into account the steric hindrance effect according to an approach made by Hayes.