Hard Microdomains Research Articles

Role of chain symmetry and hydrogen bonding in segmented copolymers with monodisperse hard segments

Thermoplastic segmented polyurethane and polyurea copolymers whose monodisperse hard segments are based on only a single diisocyanate molecule are discussed. The solid-state structure-property behavior of these materials demonstrates that a proper selection of the level of symmetry and/or cohesiveness of the hard microdomains may allow elimination of the traditional requirement of chain extension to obtain melt processable segmented urethanes, and more specifically, urea copolymers with useful structural properties.

Morphology and properties of thermoplastic polyurethane nanocomposites incorporating hydrophilic layered silicates

Hydrophilic layered silicate/polyurethane nanocomposites were prepared via twin screw extrusion and solvent casting. Good dispersion and delamination was achieved regardless of processing route, illustrating that the need for optimised processing conditions diminishes when there is a strong driving force for intercalation between the polymer and organosilicate. Evidence for altered polyurethane microphase morphology in the nanocomposites was provided by DMTA and DSC. WAXD results suggested that the appearance of an additional high temperature melting endotherm in some melt-compounded nanocomposites was not due to the formation of a second crystal polymorph, but rather due to more well-ordered hard microdomains. Solvent casting was found to be the preferred processing route due to the avoidance of polyurethane and surfactant degradation associated with melt processing. While tensile strength and elongation were not improved on organosilicate addition, large increases in stiffness were observed. At a 7 wt% organosilicate loading, a 3.2-fold increase in Young's modulus was achieved by solvent casting. The nanocomposites also displayed higher hysteresis and permanent set.

Control of structure and tack properties of acrylic pressure‐sensitive adhesives designed by a polymerization process

AbstractFree radical emulsion polymerization of methyl methacrylate (MMA) and 2‐ethylhexyl acrylate (EHA) results in the synthesis of pressure‐sensitive adhesives (PSAs) with good tack properties. Management of both the copolymer composition and the polymerization process allows one to control the behavior of the PSA. Semicontinuous (SC) processes create polymer particles whose instantaneous composition is close to that of the feed particle The SC Mixture process (continuous feeding with comonomer blends) affords nearly homogeneous latex particles and PSA films. The SC Gradient process (separate feedings at inversely varying rates) affords heterogeneous particles and films. The Batch process leads to somewhat heterogeneous films, but the hard (MMA‐rich) microdomains are made compatible with their soft (EHA‐rich) matrix because of the assumed formation of tapered‐type copolymers. Tack measurements indicate the importance of the particle and film structures. Too much hardness or softness leads to unacceptable lacks of adhesion and cohesion, respectively. Homogeneous structures prove adequate, but their tack properties collapse with rising temperature. Heterogeneous structures, with extensive phase segregation, prove unsatisfactory because they lack adhesion and cohesion. Finally, the association of well‐balanced composition and compatible heterogeneity is the criterion for suitable PSA behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2749–2756, 2003

Understanding tapping-mode atomic force microscopy data on the surface of soft block copolymers

In this paper, we focus on better understanding tapping-mode atomic force microscopy (AFM) data of soft block copolymer materials with regard to: (1) phase attribution; (2) the relationship between topography and inside structure; (3) contrast-reversal artifacts; (4) the influence of annealing treatment on topography. The experiments were performed on the surface of poly(styrene–ethylene/butylene–styrene) (SEBS) triblock copolymer acting as a model system. First, by coupling AFM with transmission electron microscopy (TEM) measurements, the phase attribution for AFM images was determined. Secondly, by imaging an atomically flat SEBS surface as well as an AFM tip-scratched SEBS surface, it was confirmed that the contrast in AFM height images of soft block copolymers is not necessarily the result of surface topography but the result of lateral differences in tip-indentation depth between soft and hard microdomains. It was also found that there is an enlarging effect in AFM images on the domain size of block copolymers due to the tip-indention mechanism. Thirdly, based on the tip-indention mechanism, tentative explanations in some detail for the observed AFM artifacts (a reversal in phase image followed by another reversal in height image) as function of imaging parameters were given. Last, it was demonstrated that the commonly used annealing treatments in AFM sample preparation of block copolymers may in some cases lead to a dramatic topography change due to the unexpected order-to-order structure transition.

TEMPERATURE EFFECTED STRUCTURAL TRANSITIONS IN POLYURETHANES SATURATED WITH SOLVENTS STUDIED BY SAXS SYNCHROTRON METHOD

ABSTRACT A set of segmented polyurethanes (PU) differing in the hard-segment structure was saturated with solvents and after the equilibrium saturation was reached, put to temperature-dependent SAXS investigations. The time-resolved mode of SAXS measurements with a linear increase of temperature from −70°C to +70°C, i.e., within the temperature range between Tg of soft and hard segments, was applied. The order-order transition leading to a greater degree of order was found at higher temperatures for almost all systems investigated. Some of the PUs exhibit two kinds of microphase separated domains. The results obtained are discussed with respect to the mean-field theory of copolymers and Koberstein and Stein model for hard microdomain structure in PUs, and correlated with temperature dependence of membrane permeability in pervaporation process.

Evaluation of physico-mechanical properties of precipitated polyurethane films in medium of free radical agents

Segmented poly(ester-urethanes) (PU) elastomers based on poly(ethylene diethylene adipate)diols as a soft segment and aromatic diisocyanates in the hard segment were synthesized by a conventional method. The precipitated PU elastomers films have been degraded after a limited exposure to free radical agents. An increase of the ratio of radical agents had an increase in the hard segment content which was associated with increased hard microdomain crystallinity, hardness and improvement in mechanical properties. It is suggested that the superior mechanical performance may be related to a interconnecting hard microdomain texture by a radical cross-linking process. The present study attempts to correlate the physical–mechanical properties of the precipitated PU films with the concentration of the free radical agents. In all cases, the effect of free radical cross-linking was to increase the ultimate tensile strain.

Microdomain Morphology of Poly(urethane urea) Multiblock Copolymers

As part of our continuing study of the solid-state morphology of poly(urethane urea) segmented block copolymers, we focus in the current paper on the use of atomic force microscopy to visualize the structure of phase-separated microdomains. Free surface and bulk images were obtained from two series of PUU films, for copolymers varying in hard segment content from 14 to 47 wt %. Using a progression of AFM tapping forces, the morphology of the hard domains at the free surfaces was found to be in the form of randomly oriented cylinders with additional spherical domains, both having lateral dimensions on the order of 5−10 nm. At higher tapping forces, phase images of microtomed surfaces of relatively high hard segment content PUU copolymers appear to exhibit hard segment-rich domains at two size scales. However, images of surfaces of specimens freeze-fractured at liquid N2 temperatures (and acquired at relatively low tapping forces) reveal the larger structures to be aggregates of smaller hard microdomains.

Structured latex particles with improved mechanical properties

Structured polymer latex particles are prepared by a swelling emulsion polymerization process, in which the initial particles are first swollen by ethylenically unsaturated monomers and the polymerization of the latter is then carried out. This special polymerization process leads to multi-phase particle morphology. Instead of a thermodynamically more favorable large-scale phase-separation, we obtain multiple, near-spherical domains dispersed within the particles. TEM analysis after selective staining reveals the size and distribution of the microdomains. Dynamic mechanical analysis of the polymer films confirms the absence of a distinct, large second phase and indicates that such microdomains interfere at the molecular level with the segmental mobility of the dispersing phase. We present examples of soft polymers as the continuous phase and hard polymers as the dispersed phase. The inclusion of hard microdomains into soft continuous phase increases dramatically both the films tensile strength and elongation, which means improved cohesive strength of the polymer material. The increase in tensile strength of the polymer film correlates directly with the hardness of the dispersed phase. Improvement in tensile strength and elongation is important in a number of industrial applications of polymers, such as flexible coatings, coalescent-free paints and pressure sensitive adhesives (PSAs). Examples are presented which demonstrate the potential applications of the swelling emulsion polymerization process.

The influence of composition ratio on the morphology of biomedical polyurethanes

Two series of thermoplastic polyurethane elastomers were synthesized from 4,4′-methylenediphenyl diisocyanate (MDI), 1,4-butanediol (BDO) chain extender, and each of poly(tetramethylene oxide) (PTMO) and poly(hexamethylene oxide) (PHMO) macrodiols. The PTMO and PHMO molecular weights were kept constant at 993 and 852 g/mol, respectively. In the PTMO-based series, the composition ratio was varied between 48 and 58% (w/w) of macrodiol; 2 commercially available PTMO-based polymers were also included. These were Pellethane 2363 80A® and its harder counterpart, Pellethane 2363 55D®. In the PHMO-based series, the composition ratio was varied between 50 and 60% (w/w) of macrodiol. The materials were characterized by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Mechanical performance was also assessed by tensile testing, stress hysteresis, and hardness testing. Altering the composition ratio had a similar effect on morphology and properties for both the PTMO and PHMO-based series. An increase in hard segment content was associated with increased hard microdomain crystallinity, hardness, and stiffness. In both series, he beginning of hard microdomain interconnectivity was observed at a composition ratio of 52% soft segment. That is to say, for the processing and annealing conditions employed, macrodiol contents of 52% and below began to produce continuous, rather than discrete, hard microdomains. Pellethane 80A® was shown to have a discrete hard microdomain morphology, while Pellethane 55D® was shown to incorporate interconnecting hard microdomains. It is suggested that the superior biostability performance of Pellethane 55D relative to Pellethane 80A may be related to its interconnecting hard microdomain texture. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 937–952, 1999

The influence of composition ratio on the morphology of biomedical polyurethanes

Two series of thermoplastic polyurethane elastomers were synthesized from 4,4′-methylenediphenyl diisocyanate (MDI), 1,4-butanediol (BDO) chain extender, and each of poly(tetramethylene oxide) (PTMO) and poly(hexamethylene oxide) (PHMO) macrodiols. The PTMO and PHMO molecular weights were kept constant at 993 and 852 g/mol, respectively. In the PTMO-based series, the composition ratio was varied between 48 and 58% (w/w) of macrodiol; 2 commercially available PTMO-based polymers were also included. These were Pellethane 2363 80A® and its harder counterpart, Pellethane 2363 55D®. In the PHMO-based series, the composition ratio was varied between 50 and 60% (w/w) of macrodiol. The materials were characterized by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Mechanical performance was also assessed by tensile testing, stress hysteresis, and hardness testing. Altering the composition ratio had a similar effect on morphology and properties for both the PTMO and PHMO-based series. An increase in hard segment content was associated with increased hard microdomain crystallinity, hardness, and stiffness. In both series, he beginning of hard microdomain interconnectivity was observed at a composition ratio of 52% soft segment. That is to say, for the processing and annealing conditions employed, macrodiol contents of 52% and below began to produce continuous, rather than discrete, hard microdomains. Pellethane 80A® was shown to have a discrete hard microdomain morphology, while Pellethane 55D® was shown to incorporate interconnecting hard microdomains. It is suggested that the superior biostability performance of Pellethane 55D relative to Pellethane 80A may be related to its interconnecting hard microdomain texture. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 937–952, 1999

The effect of thermal annealing on the thermal properties and molecular weight of a segmented polyurethane copolymer

AbstractThe influence of thermal annealing on molecular weight, microphase mixing, and multiple melting behavior of a segmented block copolyurethane is reported. The material studied contained 55% of hard segment consisting of 4,4'‐diphenylmethane diisocyanate and butanediol, and a poly (propylene oxide) diol of molecular weight 2000 as the soft segment. The thermal stability of the materials was influenced greatly by the order‐disorder transition, estimated to occur at ca. 191°C. Upon annealing above this temperature, molecular weight increased rapidly as a result of chain branching reactions. Microphase separation increased under these conditions, while the degree of hard segment crystallinity decreased. Annealing below the order‐disorder transition temperature resulted in relatively small molecular weight increases for short annealing times, but large increases for annealing times greater than one hour. Glass transition temperature data for these thermal treatments was consistent with upper critical solution temperature behavior and selective solubility by hard segment sequence length according to the Koberstein‐Stein hard microdomain model. The critical hard segment sequence for segregation was estimated (for 30 min annealing) to contain ca. 5 diisocyanate residues at 80°C, ca. 8 residues at 185°C, and increased slowly with annealing time. © 1994 John Wiley & Sons, Inc.

Small‐angle scattering analysis of hard‐microdomain structure and microphase mixing in polyurethane elastomers

AbstractThe microdomain structure of a series of segmented polyurethane block copolymers is characterized by small‐angle x‐ray and neutron scattering analyses. The materials contain hard segments formed from 4,4′‐ diphenylmethane diisocyanate (MDI) and butanediol (BD), and range in hard‐segment content from 20 to 80% by weight. The results provide evidence for a transition from discrete to continuous hard‐microdomain morphology as the hard‐segment content is increased above ca. 50%. The measured concentration dependences of the interdomain spacing, specific interfacial area, diffuse microphase boundary thickness, and scattering invariants are used to examine the validity of present models for hard‐microdomain structure. The observed behavior corresponds well with the general predictions of a lamellar model wherein partially coiled hard‐segment sequence configurations are allowed. The thickness of the hard microdomains extracted from the model corresponds to approximately four hard‐segment repeat units. Scattering invariant calculations are used together with determinations of the soft‐microphase glass transition temperatures to examine possible models for microdomain mixing. These calculations suggest that both the hard and soft microphases are phase mixed.