Order in amorphous polystyrenes as revealed by electron diffraction and diffraction microscopy

Ordered regions ranging in size between 15 and 45 Å have been established in several atactic polystyrenes of various molecular weights (4800; 51,000; and 1,800,000) as well as in an amorphous isotactic polystyrene by means of electron diffraction and high-resolution diffraction microscopy. The order is due to long-chain polymer molecules which tend to align more-or-less parallel to one another with a constant average spacing. The orderly chain packing can be disturbed upon cross-linking by electron irradiation, the disturbance being manifested in the irreversible changes in the d spacing and broadening of one of the diffuse rings which is inter-molecular in origin. From the results obtained, the high-resolution diffraction microscopy technique appears to have the potential for resolving the question regarding the presence of order in unoriented amorphous polymers. The technique can also differentiate between a broadened diffuse ring due to small crystallites and another due to paracrystals containing numerous defects.     

Digital processing of X-ray diffraction patterns from oriented fibers

Accurate measurement of intensity data from x-ray diffraction patterns of oriented fibers is one of the major difficulties in the determination of polymer structures. Accurate diffraction data are particularly important when discrimination between different, but similar, molecular models is required. Although digital processing of diffraction patterns has provided more accurate data than was obtainable by traditional measurements, many of the methods used have been partly subjective and restricted to well-resolved diffraction patterns. Background is a major source of error in intensity measurement. A general method of two-dimensional background removal has been developed. Objective methods of measuring the intensities of Bragg reflections from x-ray diffraction patterns of polycrystal-line fibers are described. A profile fitting method (which is an extension of the previously reported angular deconvolution procedure) for measuring diffraction data from noncrystalline oriented polymers is also presented. These methods have been used to collect data from diffraction patterns of a variety of nucleic acid polymers.     

On the origin of the intermediate component in the positron lifetime spectra in polymers

We have carried out an investigation of the nature of the 1 ns intermediate component in the positron time annihilation spectra of five polymers (atactic and isotactic polypropylene, polymethylmethacrylate, teflon, and polyethylene), using the magnetic quenching technique at high fields (1.5–3 T), coupled to lifetime spectroscopy. The results indicate that this component comes from the decay of a positron-electron bound system, different from that associated with the longest lifetime component. Therefore, two different Ps-like systems must be considered in the investigated polymers.     

Helical signature motif in the fibre diffraction patterns of random-walk chains

Helical geometry is one of the most dominant structural features in polypeptides, nucleic acids, carbohydrates and long-chain polymers in general. Structures of a large number of helical biomolecules and important non-biological polymers have been determined by fibre diffraction and helical diffraction theory. This paper reports on a study of fibre diffraction patterns calculated for two different on-lattice random-walk models of polymers, with no built-in helical features. It is noted that such on-lattice random walks are natural models for polymers with fixed monomer geometries and inter-monomer angles. The presence of layer-line intensities is observed, characteristic of fibre diffraction patterns from helices with an integral number of units per turn. It is shown that under certain circumstances, fibre diffraction patterns of helical objects may be difficult to distinguish from cylindrically-averaged fibre diffraction patterns of random walks on lattices with fixed angles. A simple correspondence is demonstrated between the parameters of a helix and a random-walk chain with equivalent fibre diffraction patterns. These results call for a critical examination of the way the helical diffraction theory is typically used: certain structures that have been modelled as helical might, under some circumstances, be more naturally described as random-walk chains with no preferred conformation even on the shortest length-scale and in the context of a fibre.     

A structural model for the amorphous state of polymers: Folded-chain fringed micellar grain model

A more realistic, two-phase structural model is proposed for the amorphous solid state of polymers. The model is based on structural evidence reported to date, in particular the evidence from our own electron diffraction and microscopy studies of crystallizable and non-crystallizable amorphous polymers, including near-molten rubbers. This model is different from either Kargin's chain-packet or Hose-mann's paracrystalline model. The two major elements of the model are the grain, which consists of an ordered domain and a distinct grain boundary, and the intergrain region, which consists of molecules that are in a more truly random conformation. In the ordered domain (～20～40 Å) the molecular chain segments are aligned parallel to one another with a nearly constant spacing. A natural consequence of the present model is the concentration of excess free volume in the intergrain region. The amount of excess free volume has been estimated for typical polymers in their glassy amorphous state and found to be large for polymers that possess good ductility, but small for brittle polymers. Phenomena, such as the contributions of changes in entropy and energy to the rubbery elastic force, the increasingly extended rubbery plateau with increasing molecular weight, the non-Newtonian behavior of viscous flow and lamella crystallization and formation, are also discussed and appear to be consistent with the proposed two-phase granular model.     

Morphology of amorphous polymers

It has been suggested that amorphous polymers consist of small domains (30 to 100 A) in which there is local ordering or alignment of neighboring segments. Although free volume remains the primary physical parameter useful in characterizing the properties of amorphous polymers (as percent crystallinity for crystalline polymers), the distribution of the free volume, as determined by the size, interconnection, internal order, etc. of the domains is proposed to also be of importance. The paper reviews electron microscope and electron diffraction evidence for the presence of the domain structure, while also pointing out significant remaining problems.     

Homogeneous polymer blend microparticles with a tunable refractive index

We show that homogeneous polymer blend microparticles can be prepared in situ from droplets of dilute solution of codissolved polymers. Provided that the droplet of solution is small enough (<10 mum), solvent evaporation is rapid enough to inhibit phase separation. Thus the polymers that are being mixed need not be miscible, which greatly enhances the applicability of the technique. From analysis of two-dimensional Fraunhofer diffraction (angular scattering) patterns, we show that both the real and the imaginary parts of the refractive index can be tuned by adjustment of the relative weight fractions of polymers in solution.     

POLY(3-HYDROXYBUTYRATE)-BASED COPOLYMERS AND BLENDS: INFLUENCE OF A SECOND COMPONENT ON CRYSTALLIZATION AND THERMAL BEHAVIOR*

Isotactic poly(3-hydroxybutyrate) (P3HB) is an aliphatic polyester obtained by bacterial fermentation. This truly biodegradable polymer has been widely investigated, mainly with the aim to replace conventional plastics, which cause environmental pollution. To improve its properties, extensive studies have been conducted to modify it properly by copolymerization or blending with other polymers. Macroscopic properties of polymers, particularly of polymer blends, are strongly affected by their microstructure, especially by the allocation of different phases and their level of dispersion and adhesion. In nonreactive blends, the adhesion level is mainly determined by the similarity between the chemical structure of the components, and the arrangement of the phases is remarkably controlled by the crystallization conditions. The superstructure of the crystalline phases accounts for the mechanical resistance of the blend, but the composition and the distribution of the amorphous phases also play an important role, especially with respect to toughness properties. This article reviews data on thermal properties and the crystallization process of P3HB and its copolymers, both alone and in the presence of other polymers. In particular, copolymers of 3-hydroxybutyrate with 3-hydroxyvalerate and with 4-hydroxybutyrate and blends of P3HB and the copolymers with atactic poly(3-hydroxybutyrate), poly(vinyl acetate), poly(vinyl alcohol), poly(ethylene oxide), poly(methylene oxide), poly(epichlorohydrin), and poly(methyl methacrylate) are considered. Major attention is directed toward the influence of miscibility and composition on solidification kinetics and crystal structure with the objective to provide an overview of the current state of the art.

     

Effect of annealing cold-drawn polystyrene and polymethylmethacrylate below Tg

Annealing of both cold-drawn atactic polystyrene and cold-drawn poly-methylmethacrylate at temperatures ranging from 50°C to their respective T_g's and for times ranging from 1 to 6 days gradually but completely eliminates the orientation arcing from the wide-angle X-ray diffraction pattern but induces only ～25% recovery of the cold-drawn deformation (as calculated from dimensional shrinkage). Analogous studies show that the birefringence of cold-drawn polystyrene is reduced by 57% during the annealing treatment. The results are evidence for segmental molecular motion below T_g and suggest that the molecular alignment responsible for the wide-angle X-ray diffraction arcing is not necessarily the same molecular orientation required for dimensional recovery. We have tentatively interpreted these results in terms of our recently proposed structural model consisting of both ordered domains and connecting disordered regions in the amorphous polymeric solid state. Annealing below T_g can produce chain backbone motion within the disordered regions and cause disorientation of the aligned ordered domains brought about by the initial cold-draw process.     

光在大分子聚合物中的解偏和散射

文章报道了光在大分子聚合物中的前向散射和解偏现象,并用Mie散射和Fraunhofe衍射理论对其进行了解释,提出了小角度散射的光解偏理论。     

A comparative study on the effects of Co-60 gamma radiation on polypropylene and polyimide

Polypropylene (PP) and polyimide (PI), which belong to entirely different classes of polymers, are irradiated by Co-60 gamma radiation under similar doses and similar conditions in the dose range varying from 57.6 to 230.4 kGy. The radiation responses of these two polymers are analyzed by various characterizations such as Fourier transform infrared, UV–visible, energy-dispersive spectroscopy, X-ray diffraction, scanning electron microscopy and contact angle. PP shows substantial modifications in its structure and properties while in the same dose range, PI shows remarkable stability. These two different responses are interpreted in terms of physicochemical structure and properties of these polymers.     

Study of Macromolecular Chain Dynamics in Polymer Complexes by Time-Resolved Fluorescence Spectroscopy

Poly(methyl methacrylate)s labeled with the anthracene fluorophore were prepared by free radical, anionic, and coordination polymerization yielding atactic and syndiotactic polymers. Unlabeled isotactic poly(methyl methacrylate) was prepared by anionic polymerization. Time-resolved fluorescence spectroscopy was used to study polymer association in solution. The time-dependent decays of fluorescence anisotropy show that stereocomplexation causes an increase in rotational correlation times of anthracene fluorophores both embedded in the polymer backbone and attached at the end of the side chain of polymer molecules. The rotational correlation time of anthracene fluorophore in dimethylformamide as a part of stereocomplex is 11.9 and 30 ns in the side chain and embedded in the polymer backbone, respectively, and shorter than 3 ns in noncomplexing solvent.     

Mechanisms of reversible thermal deformation of oriented polymers

Reversible thermal deformation coefficients (TDCs) of oriented samples of a flexible-chain polymer (polyethylene) and of a number of rigid-chain polymers were measured in the longitudinal and transverse directions near room temperature. The same samples were used to measure the TDCs of crystallites by x-ray diffraction. The magnitudes of the TDCs of macroscopic oriented samples and of constituting crystallites and the characteristics of the thermal deformation of flexible-chain and rigid-chain polymers are compared. A conclusion is made that the mechanisms that determine thermal deformation in the longitudinal and transverse directions for the flexible-chain and rigid-chain polymers are different.     

Crystallization kinetics of PEO-alkaline perchlorate solutions observed by energy dispersive x-ray diffraction

The crystallization kinetics of poly(ethylene oxide) doped with various alkaline perchlorate salts were measured at room temperature by means of the new energy dispersive x-ray diffraction method for the phase transition. The experimental points of the transformation coordinate were fitted using the phenomenological Liquori-Tripiciano law, the parameters of which were evaluated for each case. The influence of the concentration and of the cation dimensions on the crystallization rate is discussed. Further details about the application of this nonconventional diffractometric technique to polymers are also reported, as is an intuitive model for describing the method.     

The coil → blob transition in atactic poly(styrene) films

The structure of atactic poly(styrene) in films prepared from chloroform solutions was studied by small-angle X-ray scattering and electron microscopy. It was shown that the density fluctuations in films corresponding to dilute poly(styrene) solutions are associated with the aggregates of particles with the radius R ₀ close to the hydrodynamic radius of the coil. The value of R ₀ in the films decreases with an increase in the poly(styrene) concentration due to the coil entanglement and the blob formation. The coil (its fractal dimension is D = 2) → blob (D = 3) transition leads to an increase in the density and glass transition temperature of films, which is caused by enhancing interchain and intrachain interactions of segments in the solid state of atactic poly(styrene).     

Evanescent polarization holographic recording of sub-200-nm gratings in an azobenzene polyester

I demonstrate high-resolution polarization holographic recording with evanescent waves in a thin film of an azobenzene polyester deposited directly on the hypotenuse of a highly refractive prism. A spatial frequency greater than 7000 lines/mm and diffraction efficiency greater than 1% have been achieved. It was found that diffraction efficiency increases in the dark after the writing beams have been switched off. The biphotonic effect found in other azobenzene polymers, which converts cis states of the azobenzene to trans states followed by an ordering process that is due to aggregation, is proposed as the reason for this increase in diffraction efficiency.     

基于飞秒锁模光纤激光脉冲基频光的差频产生红外光梳

报道了一种基于飞秒锁模光纤激光脉冲基频光的光纤型差频产生(DFG)红外光梳及其研制技术.基于自主研制的重频锁定200 MHz飞秒锁模掺铒光纤激光器,经啁啾脉冲光纤放大与超连续谱产生技术,优化近零色散OFS光纤(型号:OFS-980-20)长度,结合可调延时线,获得了精准同步的基频双色脉冲;以GaSe为非线性晶体,利用光整流技术,产生了可在6—10μm范围内宽带调谐的DFG红外光梳,光梳最大光谱宽度可达1.3μm.这种光纤型远红外光梳可望在分子光谱精密测量等领域发挥重要作用.     

Study of angular responses of mixed amplitude--phase holographic gratings: shifted Borrmann effect

We present theoretical results for angular responses of transmitted and diffracted beams in mixed amplitude-phase holographic gratings. Experimental results for gratings recorded in photographic emulsions and developed without a bleaching bath, with diffraction efficiencies of >20% , are also presented. The model shows an angular shift between minimum transmittance and maximum diffraction efficiency when both index modulation and absorption coefficient modulation are present. Also, the Borrmann effect was found outside the Bragg angle. This method can be extended to a study of the mechanism of image formation in materials such as bacteriorhodopsin and azo-dye-doped polymers that exhibit both types of modulations (index and absorption).     

Configurational-conformational statistics of stereoirregular poly(methyl methacrylate)s

The mean-square radius of gyration of poly(methyl methacrylate) (PMMA) with Bemoullian statistics was theoretically investigated utilizing the method of periodic condition. The dependent curve of the characteristic ratio of mean-square end-to-end distance on periodic microstructure length increases monotonously with the length and goes gradually to its asymptotic value for atactic PMMA chains (P _m = 0.5). Conformational energies E _α ～1.3 kcal mol^?1 and E _β ～-0.6 kcal mol^?1 of the two-state scheme are acceptable, from which the derived meansquare end-to-end distance and mean-square radius of gyration conform to the experimental data. The ratio ?S²?/χ of atactic PMMA (P _m = 0.2) as a function of the degree of polymerization χ first increases with increasing chain length, then passes through a maximum at χ = 30, and finally decreases to its asymptotic value, which is in reasonable agreement with the experimental measurements reported elsewhere. The ratio ?S²?/χ as a function of stereochemical composition indicates that theoretical and experimental data are in accord for isotactic and atactic PMMA, but small-angle neutron scattering (SANS) measurements are higher than the calculated values for syndiotactic PMMA.