Form Of Syndiotactic Polystyrene Research Articles

Thermal shrinkage and heat capacity of monolithic polymeric physical aerogels

Monolithic physical aerogels based on semicrystalline linear polymers are suitable for molecular separation, catalysis, and thermal insulation. These low-density materials generally exhibit thermal shrinkage in their pre-melting region. Differential Scanning Calorimetry (DSC) shows that aerogel thermal shrinkages occur with relevant step decreases of heat capacity (ΔCp < 0). For physical aerogels based on the thermodynamically stable β form of syndiotactic polystyrene (sPS), a positive heat capacity step associated with glass transition (Tg) and a negative heat capacity step associated with thermal shrinkage are well separated. As for poly (2,6-dimethyl-1,4-phenylene)oxide (PPO), which exhibits the rather unique closeness of Tg and Tm, the two phenomena are superimposed and a regular increase of heat capacity with porosity, going from positive to progressively more negative values is observed. Negative heat capacity steps are not present for thermal shrinkage of polymer foams, which exhibit high porosity but the low surface area. On the basis of the present study, simple and fast DSC scans allow accurate evaluation of temperatures of thermal shrinkage of monolithic physical polymer aerogels.

Nanoporous triclinic δ modification of syndiotactic polystyrene

For the δ form of syndiotactic polystyrene, a nanoporous triclinic modification, different from the well-established and widely used nanoporous monoclinic modification, is presented. Detailed routes to triclinic or monoclinic modifications, which involve different guest removal procedures from different δ-clathrates, are described and rationalized. Surprisingly, both triclinic and monoclinic δ modifications can be obtained by both triclinic and monoclinic δ-clathrates. The triclinic modification, with respect to the monoclinic one, presents the advantage of higher uptake of pollutants having molecular volume in the range 0.13–0.22 nm3, mainly when present only as traces in water or in air. In particular, the different molecular selectivity of triclinic and monoclinic δ modifications, favoring n-decane and n-hexane sorption, respectively, is described. This behavior is attributed to the crystalline cavities of the triclinic δ modification, which are halved in number and increased in volume with respect to those of monoclinic δ modification.

Diffusion mechanism of CO2 in a crystalline polymer membrane studied using model gases

Molecular dynamics simulation was performed for a crystalline form of syndiotactic polystyrene, the form, which may be used as a separation and storage material by using the nanoscale cavities in the crystal. The diffusion process of penetrants in the crystal was simulated using model gases, in which the bond length, , and Lennard-Jones parameters, and , were systematically varied, based on CO2 and N2. The diffusion coefficients increased with and decreased with increasing and . The behaviour was interpreted in terms of the change in crystal cell dimensions induced by the adsorption of gas molecules. The diffusion behaviour in the form is very sensitive to the lattice parameters, which are also very sensitive to gas adsorption. The results are useful for molecular design of gas separation membranes in which the gas permeability is controllable by external stimulus.

Syndiotactic polystyrene films with a cocrystalline phase including carvacrol guest molecules

ABSTRACTA monoclinic δ‐clathrate form of syndiotactic polystyrene (s‐PS) with carvacrol (a relevant natural phenolic antimicrobial) has been prepared and characterized by X‐ray diffraction. Very informative are Fourier transform infrared spectra, in particular their OH stretching region that shows a narrow peak and a broad band, corresponding to carvacrol molecules being isolated guest of the co‐crystalline phase or dissolved in the amorphous phase, respectively. Analogous spectral features allow discriminating, for many different s‐PS guests, between molecules being in crystalline or in amorphous phases. s‐PS co‐crystalline films with carvacrol molecules being prevailingly (more than 90%) guest of the co‐crystalline phase have been prepared, even for high carvacrol content (up to 10–11 wt %). The location of most antimicrobial molecules in the crystalline phase assures a decrease of desorption diffusivity of two to three orders and hence long‐term antimicrobial release. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 657–665

Clay exfoliation and polymer/clay aerogels by supercritical carbon dioxide

Supercritical carbon dioxide (scCO2) treatments of a montmorillonite (MMT) intercalated with ammonium cations bearing two long hydrocarbon tails (organo-modified MMT, OMMT) led to OMMT exfoliation, with loss of the long-range order in the packing of the hydrocarbon tails and maintenance of the long-range order in the clay layers. The intercalated and the derived exfoliated OMMT have been deeply characterized, mainly by X-ray diffraction analyses. Monolithic composite aerogels, with large amounts of both intercalated and exfoliated OMMT and including the nanoporous-crystalline δ form of syndiotactic polystyrene (s-PS), have been prepared, by scCO2 extractions of s-PS-based gels. Also for high OMMT content, the gel and aerogel preparation procedures occur without re-aggregation of the exfoliated clay, which is instead observed for other kinds of polymer processing. Aerogels with the exfoliated OMMT have more even dispersion of the clay layers, higher elastic modulus and larger surface area than aerogels with the intercalated OMMT. Extremely light materials with relevant transport properties could be prepared. Moreover, s-PS-based aerogels with exfoliated OMMT could be helpful for the handling of exfoliated clay minerals.

Effect of chain-length of n-alkane on solvent-induced crystallization and solvent exchange phenomenon in syndiotactic polystyrene

The effect of molecular size and the vapor pressure of a series of n-alkanes as a solvent on solvent-induced crystallization of amorphous sPS and also the solvent exchange phenomenon in the δ form of syndiotactic polystyrene (sPS) were investigated by means of FT-IR spectroscopy and X-ray diffraction. The crystallization of amorphous sPS was found to be very much influenced by the molecular size and the vapor pressure of the n-alkanes used. At room temperature, n-alkanes with six and seven carbon atoms crystallize the sPS into the δ form, whereas the longer n-alkanes did not induce the crystallization of the amorphous sPS. By increasing the crystallization temperature to 50 °C, the vapor pressure of n-alkanes increases and as a result n-alkanes with eight to ten carbon atoms crystallize the amorphous sPS into the γ form unlike the cases of n-hexane and n-heptane. On further increasing the chain-length of n-alkanes to n-tridecane and n-hexadecane, no crystallization of amorphous sPS was observed even at 50 °C. By keeping the crystallization behavior in mind, we used these n-alkanes to exchange the existing solvent in the δ form of sPS/chloroform complex. n-Alkanes with six and seven carbon atoms easily replace the chloroform enclosed in the crystal lattice at room temperature and the d 010 lattice spacing was found to increase according to the molecular size of the solvent used in the exchange process. n-Alkanes with eight to ten carbon atoms could also replace the chloroform enclosed in the crystal lattice at room temperature. But in this case the d 010 lattice spacing was found to be similar or lower than that of the δ form of sPS/chloroform complex and a new reflection was observed 2 θ = 6.6°, indicating the formation of the ɛ form. On the other hand, longer n-alkanes (C13 and C16) did not intrude into the cavities of the δ form at room temperature. By increasing the solvent exchange temperature to 50 °C, the longer n-alkanes (C13 and C16) also replaced the existing chloroform in the δ form and structure transformed to the ɛ form. In this way, we found that the crystallization and solvent exchange process of sPS using n-alkanes is quite complicated and depends strongly on the chain-length of n-alkane molecule.

Structural Changes during Thermally Induced Phase Transitions Observed for Uniaxially Oriented δ Form of Syndiotactic Polystyrene

The temperature dependence of the X-ray fiber diagram and polarized infrared spectra has been measured successfully for the first time for the δ form of uniaxially oriented syndiotactic polystyrene...

Thermally‐Induced Phase Transitions in the Uniaxially‐Oriented δ Form of Syndiotactic Polystyrene

AbstractThe empty δ (δe) form of uniaxially‐oriented syndiotactic polystyrene (sPS) samples were obtained by extracting the solvent molecules from the δ form of sPS and solvent complex in acetone and methanol. Temperature dependence of the X‐ray fiber diagrams starting from the uniaxially‐oriented δe and δ form has been measured successfully at various temperatures for the first time. The transition behavior was traced clearly by separating the equatorial and layer line reflections. The δe form transformed to the γ form via an intermediate form. The intermediate form is speculated to take disordered structure due to the empty cavities present in the δe form. Calorimetric studies showed an endotherm followed by an exotherm during this phase transition, which is consistant with such a speculation. On the other hand the δ form transformed to the γ form directly without passing through the intermediate form or δe form. During the δ to γ phase transition the solvent molecules evaporate through the columnar structure in a broad range of temperature, allowing the transition to occur smoothly.

Crystal Structure of Form I of Syndiotactic Poly(para-methylstyrene)

The crystalline of form I of syndiotactic poly(para-methylstyrene) is described. In this form, the polymer chains assume a s(2/1)2 helical conformation and are packed according to the space group P2 1 /a in a unit cell with a = 24.5 A, b = 12.4 A, c = 8.1 A, and y = 143.5°. The proposed model may be described by a stacking of layers of alternating right- and left-handed helical chains efficiently packed in an interdigital structure in which phenyl rings belonging to one chain interlock with the phenyl rings of the two adjacent chains of opposite chirality. A disorder in the correct succession of the layers is suggested. A possible similarity between this form and the nanoporous 6 form of syndiotactic polystyrene, suggested by the fact they are both obtainable by guest removal from a class clathrates, is analyzed.

Guest Exchange with n-Alkanes and Host−Guest Interactions in the Clathrate Phase of Syndiotactic Polystyrene

The influence of guest size on the guest exchange behavior of the δ form of syndiotactic polystyrene was investigated using a series of n-alkanes from n-hexane to n-decane as a guest. The bands due to sorbate and desorbate molecules and the conformational sensitive bands of sPS were followed by ATR-FTIR spectroscopy. The guest exchange of the δ form containing chloroform proceeded smoothly for all n-alkanes, but it became slow with the length of n-alkane. Although the amount of TTGG helices characteristic of the clathrate structure was kept during the exchange with n-hexane and n-heptane, a decrease in conformational regularity was observed in the beginning of the guest exchange of longer n-alkanes. The absorbed amount of n-alkanes decreased with the amount of the TTGG helices. The frequency shift of the methyl asymmetric stretching band of n-alkanes suggested that the packing of n-alkane molecules in the cavity became tight as the chain length increased.

A chloroform transducer based on sPS-δ-coated quartz-crystal microbalance for gaseous environment

A prototype of a quartz-crystal microbalance-based transducer is proposed for in-situ measurements of very low chloroform activity in a gaseous environment. The sensing element consists of an innovative semicrystalline form of syndiotactic polystyrene sprayed as a thin film on the piezoelectric quartz electrode of the microbalance. Chloroform sorption and desorption in the film give rise to a mass variation, implying a related shift in the resonance frequency of the quartz. Results of static and dynamic metrological characterization of the transducer prototype at varying chloroform concentration and temperature in a gaseous environment are reported. Satisfactory sensitivity, repeatability, and dynamic hysteresis are highlighted.

Supercritical Fluid Assisted Crystal Transition of γ‐Form Crystal in Syndiotactic Polystyrene

AbstractSummary: In different media in the range of approximately 160–240 °C, the γ form of syndiotactic polystyrene was transformed into different crystal forms having different stabilities. Compared with the transition to less stable forms in ambient air and gaseous CO2, the γ form transformed into β′ form in supercritical CO2 at 240 °C and 12 MPa, and into the more ordered β″ form in supercritical CO2 and acetone at the same temperature and pressure.WAXD patterns of syndiotactic polystyrene samples treated from the γ form at 240 °C under the conditions indicated.imageWAXD patterns of syndiotactic polystyrene samples treated from the γ form at 240 °C under the conditions indicated.

Selective Guest Uptake from Solvent Mixtures in the Clathrate Phase of Syndiotactic Polystyrene

AbstractSummary: Selective absorption uptake during the guest‐exchange processes in the δ form of syndiotactic polystyrene (sPS) was confirmed by IR spectroscopy. When films of the δ form were immersed in hexane/decane or chloroform/decane mixtures, decane molecules were incorporated preferentially in the δ form. Sorbate uptake by the δ form was greatly accelerated when the sorbate was mixed with a solvent penetrable to the amorphous region of sPS.Concentration changes in a δ‐form film during sorption processes: sorption of a chloroform(D)/decane(H) mixture (1:1 molar ratio).magnified imageConcentration changes in a δ‐form film during sorption processes: sorption of a chloroform(D)/decane(H) mixture (1:1 molar ratio).

Structure and Properties of the Mesophase of Syndiotactic Polystyrene IV. Release of Guest Molecules from δ Form of Syndiotactic Polystyrene by Time Resolved FT-IR and WAXD Measurement

Syndiotactic polystyrene(sPS) forms clathrates with organic solvent molecules between helical chains of trans–trans–gauche–gauche(TTGG) conformation, so called δ form complex. By annealing or extraction, the δ form transforms to the mesophase which has highly regulated molecular cavities between helical chains. Time resolved FT-IR spectra of the sPS films containing δ form complex with toluene were obtained to clarify the relationship between the release of toluene from the δ form and conformational changes of sPS backbone chain. From the results of FT-IR, content of TTGG conformation decreased with annealing time, and increased with extraction time by acetone, indicative of the formation of γ form from amorphous phase. The mesophase extracted holds high content of TTGG conformation and is stable, compared to annealed sample. The solvent extraction method is thus a better technique to make mesophase membranes containing molecular cavities.

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.

Crystal Structure of the Clathrate Form of Syndiotactic Poly(m-methylstyrene) Containing Carbon Disulfide

The crystal structure of the clathrate form of syndiotactic poly(m-methylstyrene) containing carbon disulfide is presented. The structure is characterized by polymer chains in s(2/1)2 helical conformation and CS2 molecules packed in an orthorhombic unit cell with axes a = 17.8 A, b = 13.1 A, and c = 7.8 A, according to the space group Pcaa. The calculated crystalline density is 1.14 g/cm3 for two polymer chains (eight monomer units) and four carbon disulfide molecules included in the unit cell. As in the packing model proposed for the crystal structures of the β class of the syndiotactic poly(p-methylstyrene) clathrates, the guest molecules occupy cavities delimited by isomorphous helices along the b axis and are related by the crystallographic 2-fold screw axis of the unit cell. The packing of the helices along the a axis, instead, is similar to the close packing realized by the chains only in the clathrates and in the δ form of syndiotactic polystyrene.

Structure and properties of the mesophase of syndiotactic polystyrene. II. Effect of stepwise extraction on the preparation of the mesophase

AbstractIn this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003

Crystalline Transitions of the Clathrate (δ) Form of Syndiotactic Polystyrene during Heating: Studies Using High-Temperature X-ray Diffraction

The structural changes occurring during heating in a syndiotactic polystyrene (sPS)−solvent complex were monitored in situ by X-ray diffraction. The room temperature δ form transformed into the γ form on heating above the glass transition temperature of the sPS. The transition temperature showed a linear dependency on the amount of solvent absorbed; the higher the solvent molecules absorbed, the higher the transition temperature. However, the transition temperature does not depend on the nature of the solvent. The emptied clathrate form transformed into the γ form at the glass transition temperature. The γ form transformed into the α‘ ‘ form at ∼200 °C on heating and is independent of the δ to γ form transition. Calorimetric studies showed an endotherm followed by an exotherm during these transitions and indicated that the transitions are first order in nature.The studies provided information on the stability of various crystalline forms of sPS.

Models of Adjacent Re-Entry Folds in the β Form of Syndiotactic Polystyrene. Conformational and Packing Analysis by Molecular Mechanics

The adjacent re-entry folds of chains of syndiotactic polystyrene crystallized in the β form have been investigated by molecular mechanics. Various models of fold of chains along bilayers have been found. The results are in agreement with the literature experimental data indicating that the fold surface is irregular. Both the conformational and the packing energy of folded chains have been minimized by various techniques using several set of potential functions. A theoretical prediction of the work of fold is given.

Crystal structure of the clathrate δ form of syndiotactic polystyrene containing 1,2-dichloroethane

The crystal structure of the clathrate δ form of syndiotactic polystyrene containing 1,2-dichloroethane is presented. Polymer chains in s(2/1)2 helical conformation and 1,2-dichloroethane molecules are packed in the monoclinic unit cell with axes a=17.11 Å, b=12.17 Å, c=7.70 Å and γ=120° according to the space group P2 1/ a. The crystalline density of the clathrate δ form is 1.23 g cm −3 with two chains (eight monomer units) and two 1,2-dichloroethane molecules inside the unit cell. Molecules of 1,2-dichloroethane occupy isolated cavities between the benzene rings of adjacent polymer chains and are essentially only in the trans conformation. The location and the conformation of the 1,2-dichloroethane molecule are such that both chlorine atoms are sandwiched between two benzene rings of the same polymer chain.