Understanding the theory and practice of Tgs.

In today's computerized world, the science of materials has brought about dramatic changes unthinkable only several decades ago. These were made possible by major developments in the field of polymers and related composites, an area which has seen an outburst of activities and technological breakthroughs outstripping by far those for metals and ceramics. If we visualize a triangle having at its left and right corners one of these two materials and at the apex polymers, the total cumulative knowledge has drastically shifted toward polymeric materials. Due to the above developments and our highly competitive business environment, rubber compounders and polymer technologists are continuously striving to come up with new and improved materials offering easy processing for high performance end uses at the lowest possible cost. To accomplish this goal, it appears desirable to refresh and update our understanding of those principles which define the glass transition temperature (Tg) of polymers, since its value is critical to establish the temperature range in which a pertinent material can be used effectively.

Fundamental concepts

The reader who has had organic chemistry knows that low molecular weight cyclic compounds (phenol, etc.) are generally powders which can be purified via crystallization from their solutions - and those tiny white crystals have a consistent sharp melting point. We may also recall that in a homologous series of paraffinic hydrocarbons with the general formula [C.sub.n.H.sub.2n]+2, the melting points rise with increasing molecular weight. Furthermore, there is a gradual transition from the gaseous, liquid and solids states as shown in figure 1.

[FIGURE 1 OMITTED]

For certain linear vinyl polymers up to an average molecular weight (mw) of 15,000, this correlation is valid only for some non-thermal physical properties such as tensile strength and solution viscosities, both increasing with rising mw. To the dismay of earlier researchers, upon gradual heating similar amorphous linear polymers became brittle and charred at desirable high mw, thus precluding further processing into useful end products. Subsequently, this problem was solved by the use of plasticizers. That brittle point temperature was called "second order transition temperature." Interestingly, other polymer properties such as refractive index, electrical conductivity, etc., showed similar drastic changes when plotted against rising temperatures. In particular, the specific heat increased substantially above "the second order transition temperature" which implied that the molecules may have acquired another way of storing energy. Linus Pauling suggested that this new way of storing thermal energy is made possible by the rotation of segments within the polymer chain.

All liquids, glass, the majority of elastomers and some common plastics, with the exception of various grades of polyethylene, are classified as noncrystalline or amorphous materials. Technically, liquids of practical importance are considered fluids (i.e., they flow under their own mass) which can become very thin (used motor oils), very viscous (metals as they approach melting) or even solid without crystallizing.

At high temperatures, glasses form true liquids. The atoms have freedom to move around and respond to shear stresses. When a commercial glass is supercooled at its liquid temperature, there takes place a thermal contraction caused by atomic rearrangements that produce more efficient packing of the atoms. If this contraction, shown in figure 2, is followed by more extensive cooling, there is an abrupt change in the expansion coefficient. Below a certain temperature called glass transition temperature, Tg, there are no further rearrangements of the atoms and the only contraction is the result of smaller thermal vibrations. The temperature-volume relationship shown in figure 2 was first observed for silicate glasses.

[FIGURE 2 OMITTED]

However, it soon became apparent that these characteristics have major significance in polymeric materials. Below the Tg, polymers are hard and brittle, have low dielectric constants, etc. Above the Tg, these materials become flexible and even rubbery with concurrent changes in electrical and other physical properties.

The reason for this similar behavior can be explained by the fact that even though polymeric molecules are entangled, there is a continuous rearrangement within a polymer liquid due to thermal agitation of molecular segments. Because of this phenomenon, there has to be some "free space" in the liquid. As the temperature drops, the thermal agitation lessens and there is a decrease in both the "free space" and the vibrational amplitude. As a further illustration, many partially crystalline polymers are supercooled liquids between the melting temperature Tm and their glass transition temperature Tg (figure 3b) and are subsequently subjected to viscous flow in addition to elastic deformation. Although at this stage, the rate of deformation is relatively slow due to the high viscosity, this facilitates deformation under the influence of a long term load, as well as sufficient polymer flow within a mold at high temperature - a fact of significant practical importance.

Figure 3a shows the shear stress, [tau], required to produce unit deformation (100%) in a specimen from a commercial PMMA (Plexiglass) as a function of temperature. It can be seen that a marked change occurs just above 100[degrees] C. This temperature corresponds to the glass temperature Tg of figure 3b. As we recall, the molecules have the freedom to fold and unfold via thermal agitation above Tg. Below that temperature, there is insufficient thermal agitation to permit molecular rearrangements into close packing. This represents a discontinuity in the thermal behavior of the material. Returning to figure 3a, we note that the stress required for a deformation changes by more than two orders of magnitude at the glass transition temperature, underscoring the importance of this parameter.

[FIGURE 3 OMITTED]

Furthermore, the two curves shown in figure 3 indicate that less stress is required when the time of stressing is increased from 36 seconds to one hour. The two curves also indicate that during the same period, the Tg drops by approximately 10 degrees. This change is reflected in figure 3b as a drop in the glass transition temperature with slower cooling. Thus, slower cooling rates or longer times allow for some molecular rearrangement at somewhat lower temperatures.

The rubbery plateau

The different morphological-property relationships and their effect on deformation can be generally illustrated via the graphs shown in figure 4.

[FIGURE 4 OMITTED]

Here, gradual material transitions are represented for brittle, hard, tough and soft polymers as a function of their viscoelastic modulus Mve. Thus, Mve = [tau]/([[delta].sub.e] +[[delta].sub.f]) Where [tau] = shear stress; [[delta.sub.e] = elastic deformation; and [[delta.sub.f] = displacement by viscous flow.

The right end of the abscissa refers to higher temperatures and/or longer times, both of which introduce more deformation, and thus lower values for Mve. At the left end and below the glass transition temperature Tg, where only elastic deformation can occur, the material is comparatively rigid (a clear plastic ruler). In the range of the glass temperature, the material is leathery, it can be deformed and even folded, but it does recover quickly to its original shape. In the rubbery plateau, polymers deform readily but quickly regain their original shape if the stress is removed. A rubber ball and a polyethylene squeeze bottle exemplify this behavior because they are soft and highly elastic. At still higher temperatures, or under sustained loads, polymers show extensive deformation by viscous flow (curve 1). A highly crystalline polymeric material (curve 2) does not have a glass temperature. Therefore, it softens more gradually as the temperature increases, until the melting temperature Tm is approached. The higher density polyethylene grades with approximately 50% crystallinity lie between curves (1) and (2).

The behavior of crosslinked polymers, like cured rubber, is represented by curve (3). It can be seen that the effects of crosslinking carry beyond the melting temperature into a liquid state.

In this context, a thermosetting network condensation polymer like phenol-formaldehyde may serve as an example for extreme crosslinking, since the 3-D amorphous structure carries well beyond any imaginable melting temperature with the end result of a charred material. Curve (4) represents elastomeric materials, known for their stress-strain recovery properties. Once the glass temperature is exceeded, the recovery tendency increases with the greater thermal agitation at higher temperatures. For this reason, curve (4) rises slightly to the right across the rubbery plateau. Finally, the elastomer reaches a temperature at which it becomes a true liquid and then flow proceeds rapidly.

With the above general background, one is tempted to ask: What is the present status of a glass transition theory? According to a comprehensive review by Sperling (ref. 1), the reader is given a choice between the free volume theory,the kinetic theory and the thermodynamic theory. Quoting this realistic author, "These three theories may at first appear as different as the proverbial three blind men's description of an elephant," while they actually examine three aspects of the same phenomenon.

From what we have learned so far, the free volume theory - providing relationships between coefficients of expansion below and above Tg, while correlating viscoelastic motion to the variables of time and temperature - seems adequate and easy to remember for all practical purposes. For the sake of completeness, it should be mentioned that so far, attempts to come up with a single unifying theory have only received partial acceptance due to certain ambiguities which have yet to be resolved.

Factors affecting a polymer's Tg

The free volume of the polymer

The free volume of the polymer, Vf, can be defined as the cumulative volume of vacancies or empty spaces within a network, as illustrated schematically in figure 5. The following relationship applies for the free volume: Vf = V- Vs (2) where: V = specific volume of polymer mass; Vs = volume of solidly packed molecules.

[FIGURE 5 OMITTED]

The higher the Vf, the more room the molecules will have in which to move around and the lower the Tg. Conversely, higher Tg values were recorded for highly compressed identical polymers. Since high pressures reduce V, and Vs does not change substantially, Vf is reduced.

Bond interactions

Polymer chains that do not have freedom to easily undergo bond rotation and pass through the glass transition generally melt with difficulty, resulting in high Tgs. Likewise, highly attractive secondary forces due to hydrogen bonding or highly polar groups tend to increase the likelihood of crystallization, thus restricting the mobility of amorphous chains. This translates into higher melting temperatures, which ultimately lead to higher Tg values. For instance, polyacrylonitrile has a Tg which is higher than its degradation temperature because of extremely strong attractive forces between chains. In contrast, polymer chains which exhibit internal mobility - where bulky substituents are positioned as far away from each other as possible - are characterized by lower Tg values. For example, silicone rubber has a Tg of -120[degrees] C.

Molecular weight and branching With increasing molecular weight of a polymer, in general its glass transition temperature rises very quickly at first, then more slowly, and finally reaches a constant value. However, the kinetic chain flexibility has a major impact on the molecular weight at which the Tg of a polymer becomes constant. For instance, in the case of polyisobutylene (very flexible chains), Tg becomes constant, beginning from mw = 1,000. In contrast, for more rigid polymers with chains of low kinetic flexibility (polystyrene), Tg becomes constant at a molecular weight range of 15-40,000 and above (ref. 2).

In this context, it should be noted that high molecular weight polymers with flexible chains are characterized by relative lower Tgs and a wide rubbery plateau (-70 to +200[degrees] C). Conversely, higher molecular weight polymers with rigid chains have higher Tgs and a much narrower rubber-like elasticity range. Furthermore, the flow temperatures of linear amorphous polymers rise with increasing molecular weights.

Backbone chemical structure Three structural factors affect the value of Tg in atactic polymers. They are cohesive energy, density, molecular chain stiffness and chain symmetry.

The cohesive energy density is a measure of the strength of the intermolecular forces holding the molecules together in the liquid state. A good example of the effect of chain stiffness can be noticed by comparing the values for polymethylmethacrylate (Tg = 105[degrees] C) and polymethacrylate (Tg = 7[degrees] C). Geometrical factors come into play from structural differences where the chemical compositions are identical as in cis and trans polymers. An example of this occurs in the cis and trans polyisoprenes, where rotation about the single bonds adjacent to the double bonds is equally easy in the cis-form, but hindered in the trans configuration because of geometrical interference between methyl groups. The effect of chain symmetry can also be seen from the Tg values of polypropylene (-18[degrees] C) and polyisobutylene (-70[degrees] C). It should also be mentioned that important physical properties such as tensile strength and elongation correlate well with Tg values. Thus, higher Tg polymeric materials exhibit usually higher tensile strengths and lower elongations. The opposite was found for specimens exhibiting lower Tgs. In addition, side groups with bulky molecules, like [CH.sub.3] in polymethylmethacrylate or [C.sub.6.H.sub.5] in polystyrene, usually lead to higher Tg values. The data shown in table 1 offer a partial illustration of the effect of chemical structure on the glass transition temperature of atactic polymers.

Copolymerization

This is one of the most effective tools to create new polymeric materials. Thus, by using a variety of monomers and copolymerization techniques, it has become possible to practically "tailor" their Tgs to the desirable balance of mechanical and other properties for a broad spectrum of conventional and high performance end uses. At the same time, their processability on standard equipment was also ensured. In general, copolymers may be formed by reacting two or more monomers by addition or condensation polymerization reactions and produced as either random or stereoregular structures. A random copolymer is one in which the involved monomers, say A and B, have no definite order or configuration in the chain (A-B-B-A-B-A-A-B). A block copolymer contains polymer chains with long sequences of one monomer, followed by similar long segments of another monomer (A-A-A-B-B-B-A-A-A-B-B-B). A graft copolymer is one in which the polymer chains are made up of one monomer and the other monomer(s) is grafted as a branch chain off the main polymer chain

[ILLUSTRATION OMITTED]

The effects of copolymerization are illustrated in table 2 for SBR rubbers which are random copolymers of butadiene and styrene. Pure atactic polybutadiene is a soft rubbery solid with a Tg in the vicinity of -80[degrees] C. Pure atactic polystyrene is an amorphous brittle glass at room temperature with a glass transition temperature of about 100[degrees] C. Via addition polymerization in an emulsion of butadiene and styrene, an entire range of copolymers can be made having the approximate Tgs shown in table 2. The crystallizing tendencies of these materials also vary from the high crystallizability (on stretching) of polybutadiene to the noncrystallinity of polystyrene. According to Henderson (ref. 3), Tg is the most important determinant in the abrasion process and consequently of tire wear based on recipes for passenger tire treads. Similarly, it controls friction and consequently tire traction. It also has a strong influence on h which is related to the rolling resistance or fuel consumption caused by tires.

Typical representatives of the block copolymers are relative new materials which exhibit an unusual combination of thermoplasticity and rubber-like behavior and are thus known as thermoplastic elastomers. Although about five different types are now available commercially, since the mid-eighties the largest production volume and broadest acceptance has been gained by the polystyrene/elastomer triblock copolymers. They consist of two distinct phases, polystyrene representing the rigid one, while the elastomeric flexible domains are made up of polybutadiene and poly(ethylene-cobutylene). The pertinent designations are S-I-S; S-B-S and S-EB-S. The polystyrene end segments for separate "domains" in the continuous elastomeric phase. The resulting matrix is comparable to the network which gives strength to vulcanized rubber. Typical for these materials are two Tg values, one for the soft rubbery phase (-90[degrees] C to -60[degrees] C) the other for the rigid phase (+95[degrees] C). This distinct feature, ensuring a wide rubbery plateau, provides the following benefits: broad processing range on conventional rubber and plastics production equipment; rapid reversible transition from a melt to a solid rubber-like material which can be recycled; fast solubility of pelletized grades (S-B-S, S-I-S) in common solvents for adhesives, sealants, etc.; and good compatibility with a broad range of polymeric materials including polycarbonate, EPDM. Based on the above, it was more recently possible to overcome some of their limitations - upper service temperature and poor solvent resistance - via the development of a new class of thermoplastic elastomers by blending with polypropylene and EPDM, followed by dynamic vulcanization (high temperature mixing). It is claimed that these new TPEs are performance-wise similar or even superior to vulcanized EPDM or polychloroprene (ref. 4).

Due to their versatility and broad range of applications (adhesives, coatings, etc.), vinyl acetate emulsion copolymers - although around for some time - have not lost their popularity to this very day. The neat thing about PVAc emulsions, to begin with, is their relative low RT Tg of approximately 25[degrees] C over a broad range of mw distributions. By selecting a suitable acrylic copolymer, they are presently supplied for a spectrum of Tgs ranging from -75[degrees] C (polybutadiene dimeth-acrylate) to +110[degrees] C (cyclohexane dimethanol diacrylate), it is possible to obtain films which upon drying are harder or softer, tacky or non-tacky, as well as less or more flexible. With increasing Tgs, the films are harder and brittle, while at lower Tgs down to -75[degrees] C, they become increasingly softer, flexible and more continuous.

Blends, fillers, plasticizers and lubricants

As we know, polymers are rarely sold in their own right as plastics, but in conjunction with additives without which their properties are seriously deficient.

At one extreme is the addition of ppm of stabilizers, mold release agents and lubricants. At the other are compounds containing up to 50% fillers, plasticizers or other polymers aimed at achieving specific service requirements.

[FIGURE 6 OMITTED]

In blends there are in general three significantly different behavior patterns. First, the rare case of perfect compatibility of PPO and PS where the rheology is a rather perfect fit. Second are blends of high viscosity, rather incompatible, polymers like PVC and nitrile rubbers. The blend has a lower viscosity than either component and a different Tg. Usually the nitrile rubbers act as a high molecular weight plasticizer and reduce the high Tg of PVC. A completely different response may be observed in blends of low viscosity polymers, where the final blend may have a higher viscosity than either component and appears highly elastic.

As an illustration, the effect of various plasticizers on a vinyl-acetate resin is shown in figure 6.

[FIGURE 6 OMITTED]

As seen, increasing amounts of plasticizers cause a drop in the Tg value from approximately +63[degrees] C to as low as -18[degrees] C One can thus use Tg as a tool to assess plasticizer efficiency. Unplasticized rigid PVC can have a Tg as high as +80[degrees] C, however, by using certain plasticizers, depending upon the end use, its Tg can be reduced below zero.

Interestingly, certain nonionic surfactants of the acetylenic-diol type can reduce the Tg of acrylic top coat and primer latexes by about 9[degrees] C (ref. 6).

Determination of Tg

Transitions in polymers can be studied conveniently using DTA (differential thermal analysis); DSC (differential scanning calorimetry); TBA (torsional braid analysis); as well as dynamic mechanical, dielectric and magnetic methods. In general, each of the mentioned methods has its merits for particular applications. However, some are preferred over the others for economic and practical reasons (operator's skill, duration of measurements, etc.). For example, the torsional method would take up a day or more compared to 15-20 minutes by DTA or DSC. Due to the fact that Tg data are dependent on the rate of measurements, no two methods necessarily give the same value.

[FIGURE 7 OMITTED]

DSC (ASTM D 3418) is by far the most commonly used method for Tg determinations. A pertinent apparatus is schematically represented in figure 7.

[FIGURE 7 OMITTED]

The principle of the DSC technique involves detection of the temperature differences between the sample and a reference substance. In the DSC-50 apparatus (ref. 7), a pin wheel type high sensitivity detector is used to ensure base line stability. A servo system is used to supply energy at a varying rate so that the temperature of both materials, sample and reference, are maintained equal. As seen from the graph shown in figure 8, the DSC plots the energy supplied vs. average temperature.

[FIGURE 8 OMITTED]

Thus, in essence, DSC measures the amount of energy (heat) absorbed or released by a sample as it is heated, cooled or held at constant temperature. The areas under the peaks can be directly related quantitatively to the enthalpic changes.

The measurement of Tgs involves the following steps: * A sample cut to proper size is heated gradually in a nitrogen atmosphere to about 30[degrees] C above RT melting temperature at the rate of 20[degrees] C/min. (to avoid development of any heat history);

* a specific temperature is held for 10 min.;

* the sample is cooled to a temperature lower by 50[degrees] C than the anticipated Tg;

* a specific temperature is held for 10 min.; and

* the sample is reheated again at a rate of 20[degrees] C/min.

[FIGURE 9 OMITTED]

The glass transition temperature is determined from the heat flow vs. temperature graph (figure 9) by plotting the following temperature readings: Tf = extrapolated onset temperature [degrees] C; Te = extrapolated end temperature [degrees] C; Tm = mid-point temperature [degrees] C.

[FIGURE 9 OMITTED]

Depending upon the nature of a particular sample, certain procedural adjustments have to be made. Thus, for rubber specimens, known for their low Tg' s, an apparatus with cooling capabilities is employed using liquid nitrogen as refrigerant. In the case of a polyvinyl acetate adhesive study containing different amounts of solvent, an additional aluminum seal cell was used to suppress solvent evaporation. For coatings such as alkyd paints, DSC measurements are carded out on film specimens of various film thicknesses (0.2; 0.1; and 0.05 mm) using a metallic plate as base. Tg data revealed in this case a close correlation between film thickness, degree of polymerization and mechanical properties. Due to increased awareness of the benefits offered by DSC, instrument manufacturers have come up lately with a series of improvements such as power compensation for the DSC-7 (ref. 10). This apparatus can be used to determine accurate specific heats without running a standard reference material, thus offering a more rapid alternative to ASTM E-1356-91. In addition, the instrument's calibration constant is essentially independent of operating parameters. In a follow-up publication, Cassel (ref. 11) states that Tg can be more accurately determined if it is measured from the specific heat capacity curve of a Step Scan analysis. The Pyris I DCS embodies more advanced features such as: The sample and reference material are each confined to a separate self-contained calorimeter. Thus, when an exothermic or endothermic change occurs in a sample, power (energy) is applied or removed from one or both calorimeters to compensate for the energy change occurring in the sample. In addition, linear temperature measurement is achieved by replacing thermocouples with platinum resistance thermometers. Furthermore, this apparatus employs laminar flow techniques to create a thin curtain of air over the cold sample holder which is kept dry and frost-free. Benefits claimed are higher sensitivity, better resolution and a user-friendly operator interface with the Microsoft Windows platform via an adjustable control panel.

Not to be outdone, L.C. Thomas (ref. 12) and G. Dallas (ref. 13), representing another supplier, claim in recent publications similar benefits in terms of peak resolution and sensitivity as well as direct measurement of heat capacity. This is achieved via a faster "modulated" DSC in which a sinusoidal change in heating rate allows the operator to automatically separate the heat capacity base line from the total heat flow signal. All of the above has been made possible by a single Tzero cell designed for both heating and cooling without reciprocal negative interference.

Applications of glass transition temperature

Packaging tapes In the mid-seventies, the high end of the carton sealing tape market was dominated by polyester film (Mylar) pressure sensitive tapes, which offered a desirable balance of strength and service-related properties (cardboard tear, box drop test, etc.). The low end of this market segment was dominated by imported PVC, so called "mover tapes," which had a desirable stretchiness, via induced rubbery plateau elasticity, good cardboard tear and sufficient strength for relative small cartons with a weight not exceeding 45 to 65 kg. This situation started to change with the arrival of monodirectional oriented polypropylene film, followed by the first commercial development of pertinent packaging tapes. These, at that time, new mover tapes were able to compete head on with the imported PVC tapes on a cost/performance basis and have since gained worldwide acceptance.

One is, of course, tempted to ask, why polypropylene? The answer is simple, because of its lower specific gravity, 0.8 vs. 1.32 for the polyester PET (Mylar) and 1.3 for the PVC film in conjunction with similar RT Tgs. This difference translates into a commercially significant higher Area Factor (30,000 sq. in./lb./mil) as seen from the graph shown in figure 10.

[FIGURE 10 OMITTED]

Although the production of MD (monodirectional) PP film involves a series of proprietary techniques (melt extrusion, chill roll casting, etc.), the difference between Tm and Tg, as well as polymer molecular weight distribution, were the controlling factors leading to a satisfactory end product with a desirable balance of properties. In regard to mechanical characteristics, the PP film came close to the PVC film, but as expected, both were inferior to the PET film of identical thickness. For the bulk volume of shipped cartons and packages not exceeding the 45 to 65 kg weight range, this limitation combined with the lower cost did not affect the broad acceptance of PP carton sealing tapes.

Pressure sensitive adhesives

Concurrently with the above development, the gradual shift from solvent to waterborne pressure sensitive adhesives also took place, prompted by environmental constraints. The performance of pressure sensitive adhesives (PSAs) is, in general, dependent on the Tg of the rubber/resin blend within a specified temperature range. For the majority of end uses, PSAs will typically have a Tg ranging from -15[degrees] C to +5[degrees] C.

The selection of elastomer latexes - to act as suitable replacements - was dictated and dependent on the following properties: molecular weight, molecular weight distribution, Tg and molecular structure. For example, in SBR latexes, too much styrene, which imparts cohesive strength to the PSA, will increase the Tg of the copolymer to a point where the room temperature flow of the formulated adhesive and subsequent wetting of the substrate will be inhibited. This is especially critical for the hydrophobic PP film.

Like in the solvent-based pressure-sensitive adhesives, where the high Tg of tackifying resins is offset by the low elastomer Tg, to achieve the desirable mentioned range and formulating latitude, resin emulsions can be effectively employed for polymer latexes. The type of resin used with a particular polymer latex is determined by chemical structure and molecular weight. Figure 11 shows the effect of a rosin ester emulsion on the Tg of an acrylic latex.

[FIGURE 11 OMITTED]

As seen, increasing resin content raises the Tg of the acrylic latex. It is also possible to circumvent entirely the use of resin emulsions by designing vinyl acetate-ethylene or vinyl acetate-acrylic copolymer or terpolymer latexes which can exhibit desirable properties through their hard and soft segment molecular structure.

Construction adhesives

By and large, the formulating principles are similar to those described above. However, the emphasis is on achieving maximum specific cohesive strength upon bonding a variety of construction materials (wood, drywall, ceramic tiles, PVC, etc.). Lately, polyurethane adhesive and sealants have gained preference in certain high performance end uses over other elastomeric raw materials such as S-I-S, S-B-S block copolymers and styrene-butadiene copolymers, some partially crosslinked and supplied in crumb form. Low Tg acrylic latexes are also available for specific applications. High performance glazing sealants take advantage of the low Tgs of silicone polymers known for their outstanding weathering characteristics.

High performance coatings

Optical glass fibers for light wave telecommunications have challenged the ingenuity of polymer technologists. The problems of transmission losses due to microbending can be minimized by selecting primary and secondary fiber coating materials capable of exhibiting moduli of [10.sup.6] to [10.sup.7] Pa in the rubbery plateau, as well as low Tgs to ensure adequate performance at low field temperatures (-40[degrees] C). Silicone rubbery systems based on poly(dimethyl siloxane) Tg -119[degrees] C and poly(phenyl siloxane) Tg -59[degrees] C showed promising performance (ref 15).

Advanced testing procedures for automotive coatings involving Tg measurements and other methods provided valuable information. Thus, the level of coating degradation was assessed successfully by measuring the Tg values by DSC and the dielectric constant of the entire coating system. A good data correlation was found between outdoor Florida exposure for 60 days and UV testing for 250 hrs. (ref 16).

Another area of present interest involves the development of highly water resistant hydrophobic coatings for concrete and other cementitious substrates. Recently it was reported that by copolymerizing long chain vinyl ester monomers with butyl acrylate, it was possible to establish a direct correlation between copolymer composition and equivalent Tg values (ref. 17).

Conclusions

The viscoelastic behavior of polymeric materials is temperature dependent. At low temperatures, they form glasses without crystallization, whereas at higher temperatures they form viscous liquids. The glass transition temperature Tg defines the change from the glassy to the viscous rubbery state. Atactic elastomers and plastics are room temperature amorphous materials characterized by low Tg values. Isotactic plastics, with molecular chains of stereoregular configuration, can develop high degrees of crystallinity and high Tg values. A good example is the high Tgs of synthetic fibers. Some highly crystalline materials do not exhibit upon heating any rubbery plateau, have no Tg, and change upon heating from the glassy to the molten state. A wide rubbery plateau is desirable for compounding and processing purposes. This can be achieved via blending of elastomers with hard resins. In general, glass transition temperatures increase with higher crystallinity, crosslinking density, specific molar cohesion, mw and addition of backbone stiffening groups. A broad mw distribution may lead to lower Tgs. The relevance of this parameter for the development of high performance polymeric materials has been recognized by manufacturers of differential scanning calorimeters which exhibit ever increasing sophistication via new technologies for measuring TGs. Given the broad applications and increasing interest in many areas of research and development, this review should by no means be considered as all comprehensive but rather providing a meaningful perspective on the field.

Table 1 - effect of chemical structure on glass transition
temperature *

Atactic **                      Structure            Morphology with-
polymer                      of repeat unit            out external
                                                       stress

Poly(dimethyl siloxane)   [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polybutadiene             [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polyethylene              [MATHEMATICAL EXPRESSION   Crystallizable
                          NOT REPRODUCIBLE IN
                          ASCII]

Polyisobutylene           [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Natural rubber            [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polychloroprene           [MATHEMATICAL EXPRESSION   Amorphous &
                          NOT REPRODUCIBLE IN        Crystallizable
                          ASCII]

Polypropylene             [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polyvinyl acetate         [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polyamide 6.6             [MATHEMATICAL EXPRESSION   Crystallizable
                          NOT REPRODUCIBLE IN
                          ASCII]

Polyvinylchtoride         [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Polystyrene               [MATHEMATICAL EXPRESSION   Amorphous
                          NOT REPRODUCIBLE IN
                          ASCII]

Atactic **                Specific molar      Approx.
polymer                     cohesion       Tg[degrees] C
                          kcal/5A chain

Poly(dimethyl siloxane)          -               -123

Polybutadiene                   1.1               -85

olyethylene                     1.0               -69

Polyisobutylene                 1.2               -70

Natural rubber                  1.3               -72

Polychloroprene                  -                -50

Polypropylene                    -                -20

Polyvinyl acetate               3.2               +29

Polyamide 6.6                   5.8               +50

Polyvinylchtoride               2.6               +80

Polystyrene                     4.0              +100

* All Tg data represent averaged values from different
literature sources

** Polymers of irregular molecular configuration with random
arrangement of functional groups
Table 2 - effect of copolymer
composition on glass
transition temperature

Butadiene   -   Styrene   Approx.
                          Tg[degrees] C
100                          -80
 75         -    25          -57
 60         -    40          -38
 50         -    50          -21
 30         -    70          18
 10         -    90          68
                100         100

References

(1.) L.H. Sperling, "Introduction to physical polymer science, "John Wiley & Sons, Inc., NY, 1986.

(2.) A. Tager, "Physical chemistry of polymers," Mir Publishers, Moscow, 1978.

(3.) J.N. Henderson, "Styrene-butadiene rubbers, "p. 227 in "Rubber Technology," 3rd. Ed., edited by M. Morton.

(4.) C.P. Rader, Chemtech, 18, (1)54 (1988).

(5.) D. Sudley, "Plasticizers," p. 282 in "The Science of Surface Coatings," edited by H.W. Chatfield, Ernest Benn Ltd., London, 1962.

(6.) J. Schwartz, Paints & Coatings Industry, XVII, (5) 24 (2001).

(7.) "Stand alone thermal analysis system application to polymeric materials," I, 2, Shimadzu Corp. Int. Marketing Div., (1990).

(8.) F. Rodriguez, "Principles of polymer systems," p. 488, McGraw Hill Book Co., NY, 1982.

(9.) Same as above 7. 1, 6.

(10.) B. Cassel and M.P. DiVito, American Laboratory, 26, (I) 14(1994).

(11.) B. Cassel, American Laboratory, 32, (1) 22 (2000).

(12.) L.C. Thomas, American Laboratory, 33, (I) 26 (2001).

(13.) G. Dallas, J. Groh, T. Kelly and R. Danley, American Laboratory, 33, (8) 26 (2001).

(14.) A. Zawilinski, Tech VII, Proceedings of PSTC Seminar (1984).

(15.) L.L. Blyler and C.J. Aloisio, Chemtech, 17, (11) 680 (1987).

(16.) F. Deflorian, S. Rossi and P.L. Bonora, Journal of Coatings Technology, 72, (9) 81 (2000).

(17.) D.R. Bassett, Journal of Coatings Technology, 73 (1)43 (2001).

(18.) Modern Plastics Encyclopedia, 1992, pp. 550-554, Film and Sheet table.