Friedel-Crafts crosslinking methods for polystyrene modification: 2. Tg′Tll and T′ll transitions of grated polystyrene

Polystyrene was grafted with 1,4-dimethyl-2,5-dichloromethyl benzene in dichloroethane solution at 50°C using SnCl₄ as a catalyst. Thermal analysis of grafted polystyrene samples using differential scanning calorimetry revealed two liquid-liquid transitions, T_ll and T′_ll, in addition to the glass transition, T_g′ and the decomposition temperature, T_d. The effect of the number of junctions per polystyrene chain on T_g′T_ll and T′_ll is examined and analysed.     

The Tll (>Tg) transition of atactic polystyrene

Torsional braid analysis (TBA) (～0.3 Hz) and differential thermal analysis (DTA) data are presented for the temperature for the region 0–200°C for two series of atactic polystyrenes with narrow molecular weight distributions: (a) anionic series, M?_n = 600–2×10⁶, M?_w/M?_n ? 1.1; (b) fractionated thermal series, M?_n = 2,000–1.1×10⁵, M?_w/M?_n < 1.25. Preliminary results on bimodal blends are also reported. Heating and cooling cycles were employed with TBA; only the heating mode was used with DTA. In addition to a dynamic mechanical loss peak at T_g, a higher temperature loss peak was also found. Designated the T_ll or liquid–liquid transition (relaxation), its temperature is 1.1 to 1.2 T_g (°K) for polymers with molecular weight below the critical molecular weight (M_c) for chain entanglements. Above M_c ? 35,000, it rises steeply, being ?200°C for M?_n = 110,000. The common dependence of T_g and T_ll on M?_n^?1 below M_c suggests a common molecular origin. The two facts, (a) that T_ll > T_g and (b) that T_ll reflects chain entanglements, further suggest that T_ll involves a longer chain segment length and possibly the entire molecule. Comparison of T_ll versus log M plots with T versus log M isoviscous state plots based on zero-shear melt viscosity data from the literature implies that T_ll measured by the TBA technique corresponds to an isoviscous state of 10⁴–10⁵ poises. The employment of narrow molecular weight polymers is presumably responsible for both the linear variation of the T_ll transition with M?_n^?1 (which suggests a free volume basis for the relaxation) and the form of the variation of the T_ll transition with log M (which suggests an isoviscous basis for the relaxation). The sharpness of the T_ll loss peak by TBA decreases with increasing molecular weight and dispersity. The DTA endothermic event corresponding to T_ll is clearly related to the occurrence of flow since the fused films which result from heating granules to 200°C and cooling to R.T. do not reveal a T_ll on reheating. If a fused film is crushed, a T_ll event is observed on heating. For bimodal blends with M?_n < M_c for both components, the T_ll transition was averaged; with one component less than and one greater than M_c, the T_ll transitions of the components appeared to occur independently at temperatures corresponding to those of the isolated components. In accordance with Ueberreiter and Orthmann, T_g appears to separate a glassy state from a fixed liquid state, whereas T_ll separates the fixed liquid from a true liquid state. Possible molecular interpretations for the T_ll process are discussed. Systematic bodies of data from the literature which indicate the presence of the T_ll process in other polymers are summarized.     

Investigation of the Tll (>Tg) transition in polystyrene by torsional braid analysis

Investigation of the T_ll (>T_g) transition in amorphous poly-styrene by the technique of torsional braid analysis is reviewed. For the most part, the relaxation behaves like the glass transition (T_g) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a compatible plasticized system.     

Fourier transform infrared spectroscopic study of transitions above Tg in atactic polystyrene

Fourier transform infrared spectroscopy (FT-IR) has been applied to the study of the molecular mechanisms of transitions of atactic polystyrene above T_g. Intensity measurements of vibrational modes as a function of temperature revealed two transitions above T_g, which are designated as T_u and T′_u. T′_u is independent of molecular weight as opposed to the molecular weight dependent T_u whose behavior is similar to T_g. Infrared measurements are more sensitive to T′_u than T_u. Conformationally sensitive bands show that T′_u may be related to disruption of local order where there is a negligible barrier to conformational change.     

Effect of dispersity on the Tll (>Tg) transition in polystyrene

The presence of a high temperature (>T_g) relaxation in amorphous polystyrene has been investigated further. In the previous work,¹ the techniques of differential thermal analysis (DTA) and torsional braid analysis (TBA) were employed to study polystyrene as a function of “monodisperse” molecular weight. The occurrence of the T_ll transition appeared to be associated with the attainment of a critical viscosity level with also corresponded with a free volume level. An entanglement network developed at a critical value of molecular weight, M_c, giving a break in the T_ll-versus-M plots. The present work deals with the influence of dispersity on the T_ll transition, below and above M_c. A series of binary blends of “monodisperse” anionically polymerized polystyrenes with systematic changes in M?_n and heterogeneity index (M?_w/M?_n) was tested by TBA. The results show that when both components have molecular weights below M_c, single and average values of T_g and T_ll are observed which are linearly related to M?_n^?1, as predicted by free volume arguments. Although a single T_g is observed when one component has a molecular weight above and the other has a molecular weight below M_c, the components appear to undergo the T_ll relaxation independently. The results indicate that both the glass transition and the T_ll transition are basically governed by the same type of molecular motion but at different length ranges.     

Prediction of brittle-to-ductile transitions in polystyrene

In this study it is attempted to predict brittle-to-ductile transitions (BDTs) in polystyrene blends, induced either by an increase in temperature or by a decrease in inter-particle distance. A representative, two-dimensional volume element (RVE) of a polystyrene matrix with 20% circular voids, is deformed in tension. During deformation a hydrostatic-stress based craze-nucleation criterion [1] is evaluated. The simulations demonstrate that crazes initiate at low temperatures while a transition from crazing to shear yielding (BDT) is found around 75 °C. The numerical results correlate well with tensile tests on similar heterogeneous polystyrene. The presence of an absolute length, as experimentally found, is more difficult to explain. Near a free surface a T_g-depression is measured for polystyrene and also the resistance to indentation in polystyrene is lower than expected from bulk properties. Both observations are rationalised by an enhanced segmental mobility of chains near a free surface. As a consequence of these findings, an absolute length-scale could be incorporated in the numerical simulations. For simplicity, the length-scale is modelled by taking a temperature gradient over a thin layer near the internal free surfaces of the RVE. Deformation of the RVE with different absolute length-scales shows that indeed also the experimentally found brittle-to-ductile transition can be predicted if the ligament thickness between the inclusions (‘voids’) in polystyrene is below a critical value of ca. 15 nm.     

Solubility and diffusion of propane in blends of polystrene and poly(vinyl methyl ether) at T > Tg

The solubility and diffusion of propane in rubbery blends of polystyrene and poly(vinyl methyl ether) were investigated at low diffusant pressure. Solubibity is characterized by Henry's law and diffusion by Fick's law with concentration-independent diffusion coefficients. The composition dependence of the solubility and diffusion parameters is presented. The diffusion coefficients for propane in the blends are related to those for the component polymers by using the glass transitions as reference temperatures.     

The effects of heating extruded polystyrene above Tg in dry and in aqueous environments

Sections of an extruded PS rod have been heat-treated above T_g in both dry and aqueous environments. In both cases the sections relax and shrink in the axial direction. Raised surface features reveal the presence of localized orientation due to flow past obstructions in the extruder. The heat treatment in aqueous glycerol followed by quenching results in an internal mist-like pattern which also reflects the flow pattern past the extruder breaker plate. The aqueous diffusion is rapid and apparently Fickian. About 20% of the water present in the mist pattern is in the liquid phase.     

Investigation of the Tu(>Tg) relaxation in homopolymers and block copolymers of styrene by torsional braid analysis

Investigation of the T_u (>T_g) relaxation in amorphous polymers of styrene by the technique of torsional braid analysis is reviewed. For the most part the relaxation behaves like the glass transition (T_g) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a polystyrene homogeneously plasticized throughout the range of composition. Diblock and triblock copolymers also display a T > T_g relaxation above the T_g, of the polystyrene phase. Two results in particular suggest that the T_u relaxation is molecularly based. (1) The T_u temperature is determined by the number average molecular weight for binary blends of polystyrene when both components have molecular weights below M_c. (the critical molecular weight for chain entanglements). (2) Homopolymers, and diblock and triblock copolymers of styrene, have a T > T_g relaxation at approximately the same temperature when the molecular weight of the styrene block is equal to that of the homopolymer.     

Thermal transitions in cold-crystallized blends of isotactic and static polystyrene

Solution-cast blends of isotactic and atactic polystyrene were stripped of casting solvent and heated above the glass transition temperature to temperatures below the equilibrium melting point of the respective blends to induce the development of crystallinity of the isotactic polystyrene. The samples, maintained at these crystallization temperatures for various times, were subsequently characterized by differential scanning calorimetry. The amount of crystallinity, the crystalline melting point, and the rate of crystallization were determined for each of the blends from the d.s.c. scans. The development of crystallinity, characterized by increased melting points, resulted from annealing at progressively higher temperatures. The maximum rate of crystallization for each of the blends was observed at a temperature corresponding closely to $\text{8}\text{9}$ of the absolute equilibrium melting temperature. The decrease in the melting point of the isotactic polymer, crystallized in the presence of atactic diluent, resulted primarily from weak enthalpic interactions between isotactic and atactic polystyrene homopolymers. Scanning electron photomicroscopy revealed that atactic-rich blends, immersed in n-hexane at 40°C, developed a non-intercommunicating network of microvoids. Similar solvent treatment of isotactic rich blends had little effect on the samples. D.s.c. measurements confirmed recently published results which indicated that n-hexane, at modest temperatures, does not induce the development of crystallinity in isotactic polystyrene.     

Orientation of polystyrene chains stretched above Tg as studied by fluorescence polarization

Fluorescence polarization has been used to measure the orientation of polystyrene chains labelled by an anthracene group covalently bound in the middle of the chains and dispersed in atactic uncrosslinked polystyreme. Stretching is performed at constant strain rate and various temperatures above T_g using equipment which is described. The influence of strain rate and temperature on chain orientation is reported and leads to the conclusion that the orientation of the central part of the labelled chains is not related to the complete stress but is governed by the rubbery deformation. The occurrence of relaxation phenomena during stretching is demonstrated.     

Thermal and current noise in carbon black-filled polystyrene and polyethylene in the vicinity of Tg and Tm

Thermal and current noise in carbon black-filled polystyrene and polyethylene was studied in the vicinity of T_g and T_m. A highly conductive grade of carbon black was used (Ketjenblack EC): 4% for PS and 6.5% for PE. Pronounced maxima in noise voltage were found at T_g for PS and T_m for PE. Prolonged storage at T lower than T_g enhanced the intensity of the noise peak at T_g for PS. A hysteresis occurred on cooling after a heating cycle, with the current and thermal noise being lower on the cooling cycle (no maximum in the case of PS). The peak temperature was lower for PE on the cooling cycle. Good agreement was found between experimental and calculated thermal noise values. The thermal noise spectra were white within the temperature range of 20° to 140°C. The appearance of the current noise peaks could be associated with a conduction model where a decrease in the number of contact points in the carbon black network occurs in the transition regions. This is in accordance with the concept proposed earlier that the carbon black network existing in a polymer undergoes a rearrangement when the polymer melts (softens) or solidifies.     

Thermomechanical analysis of chain-extended PET and PBT

Two series of samples, one of PET and another of PBT, were received after chain extension at different reaction times with two new chain extenders (diimidodiepoxides). These samples showed different intrinsic viscosity and degree of branching or crosslinking. The effect of this differentiation on thermal properties was studied by thermomechanical analysis (TMA). The parameters studied were the glass transition temperature (T_g), melting temperature (T_m), and the linear expansion coefficient (α). It is remarkable that in the case of PET amorphous or semicrystalline samples, two peaks appeared next to the T_g in the TMA thermogram. The first peak appeared at a temperature very close and lower to the T_g, and the other peak, at higher temperature into the “cold crystallization region.” The presence of two such peaks was not detected in the DSC thermogram of PET samples either in the TMS or DSC thermograms of PBT. The T_g values were found to agree to within ±1°C of those obtained from DSC; on the contrary, the T_m values varied significantly from those received from DSC. The linear expansion coefficient of samples was found to increase with the degree of chain extension. © 1996 John Wiley & Sons, Inc.     

Thermomechanical analysis of binary polymer blends

Thermomechanical analysis by penetration and extension modes, was performed on polyolefin/polystyrene blends (high density polyethylene, low density polyethylene or isotactic polypropylene with atactic polystyrene) and polyolefin/polyolefin mixtures (high density polyethylene or low density polyethylene with isotactic polypropylene). All the measurements were performed on cylindrical specimens obtained directly by extrusion on which extensive mechanical and morphological studies were previously made. It was found that the addition of small quantities of a polymer to a different polymeric matrix tend to modify the thermomechanical behaviour of the whole system. Furthermore the results showed that such an analysis seems to be a suitable tool to get useful information on the thermal and morphological transitions as well as on the interactions between the components in the blends.     

Probe segregation and Tg determination of a supported ultra‐thin polystyrene film studied by X‐ray and neutron reflectivity,and SIMS

Selective deuteration combined with X‐ray and neutron reflectivity has been used to determine the extent of probe segregation to an interface in ultra‐thin polymer films as a function of the number of thermal cycles. The extent of probe segregation to the interface was also investigated using Secondary Ion Mass Spectroscopy (SIMS). Significant probe segregation was not evident in these studies. X‐ray reflectivity has also been employed to measure the coefficient of thermal expansion of ultra‐thin polystyrene films supported on quartz. The glass transition temperature determined with this method is equivalent to that measured for bulk polystyrene films.     

Infrared studies of cryogenic transitions in poly(ethylene terephthalate) and polystyrene

Infrared absorbance measurements as a function of temperature down to 13°K have been made on poly(ethylene terephthalate) and polystyrene. Transition temperatures are assigned to the temperatures where there are abrupt changes in the absorbance or integrated band intensities. Bands of known structural origin arising from either crystal-line or amorphous phases are studied to probe the structure basis and environment effect for each transition. Transitions are found around 50, 120, 180, and 220°K for PET and around 50° and 235°K for polystyrene.     

Thermomechanical analysis of fired ceramic materials

A thermomechanical analysis of fired clay specimens is carried out and empirical equations are derived for dilatation (shrinkage) as a function of temperature. The experimental results obtained can be used in designing optimum firing technology for preparing ceramics from polymineral clays.Translated from Steklo i Keramika, No. 11, pp. 26–27, November, 1996.     

On the reproducibility of Tg measurements on polystyrene using d.s.c.

In determining the reproducibility of T_g data of a standard polystyrene measured by differential scanning calorimetry (d.s.c.), the effects of d.s.c. trace interpretation method, extrapolation to zero scan rate, polymer sample form, a thermal treatment and a thermal cycling procedure on the results were examined. The results showed that data from scanning at 8°C/min are preferred to those from extrapolation to zero scan rate while the overall reproducibility was unaffected by the d.s.c. trace interpretation method used. However, the onset of the transition step was selected to define T_g in this work. Similarly, no differences in the reproducibility of data were noted between the overall results for granular or sheet samples, although certain treatments of specific samples did produce changes in some instances. The mean value for T_g at 8°C/min was found to be 102.3°C. Imposing the thermal treatment on the samples did not significantly alter the reproducibility of the results whereas in contrast, cycling markedly improved the reproducibility of sheet sample data but left that of granules unaffected.