analysis has a long and prominent role in the characterization of materials,
including polymeric materials. Kinetic studies in one form or another have
often been employed in an attempt to assess stability, predict lifetime, establish
degradation pathway, or project suitable processing conditions. The results
of such studies have often formed the basis for the proposal of the ‘mechanism’
of reaction. This despite the fact that the reaction being observed is often
unknown or is not a single process but rather several parallel or consecutive
events. This latter is particularly true for ‘variable temperature kinetics’.
The utility/value of such exercises is marginal at best and contributes nothing
to an understanding of the mechanism of any of the reactions involved.
chloride copolymers containing a predominance of vinylidene chloride (85-90%)
have long been important barrier polymers widely used in the plastics packaging
industry. These materials display excellent barrier to the ingress of oxygen
and other small molecules (to prevent food spoilage) and to the loss of food
flavor and aroma constituents (to prevent flavor scalping on the supermarket
shelf). While these polymers have many outstanding characteristics, which
have made them commercial successes, they tend to undergo thermally-induced
degradative dehydrohalogenation at process temperatures. The dehydrochlorination
occurs at moderate temperatures (120-200C) and is a typical chain
process involving initiation, propagation and termination phases. Defect structures,
namely internal unsaturation (allylic dichloromethylene groups), serve as
initiation sites for the degradation. These may be introduced during polymerization
or during subsequent isolation and drying procedures. If uncontrolled, sequential
dehydrohalogenation can lead to the formation of conjugated polyene sequences
along the polymer mainchain. If sufficiently large, these polyenes absorb
in the visible portion of the electromagnetic spectrum, and give rise to discoloration
of the polymer. The dehydrochlorination process may be conveniently monitored
by thermogravimetric techniques. Both initiation and propagation rate constants
may be readily obtained.
Thermogravimetry is often used to study polymer degradation. Most often the information obtained may have some practical application
but is of limited value for the determination of fundamental processes which may be occurring. A kinetic expression or activation
parameters for a complex process which may involve consecutive or parallel reactions provides almost no information about
any of the reactions that might be occurring. However, for single, well-defined processes, thermogravimetry, in conjugation
with other analytical methods, can be effectively utilized in the determination of reaction mechanism. The thermal degradation
of vinylidene chloride barrier polymers corresponds to the elimination of hydrogen chloride initiated at an allylic dichloromethylene
unit in the mainchain. This process is uncomplicated by competing reactions.
Thermogravimetry may be utilized to obtain meaningful rate constants and activation parameters for the degradation. This in
conjunction with mass spectral analysis of evolved gas, characterization of both the polymer and degradation residue by ultraviolet,
infrared and 1H and 13C NMR spectroscopy, and the study of model compounds has permitted a detailed description of the degradation process. General
purpose poly(styrene) is a commodity polymer widely used in the food packaging industry as well as many others. If processed
at excessively high temperature, it undergoes thermal degradation to expel styrene monomer which can impart negative flavor
and aroma characteristics to packaged food items. The degradation reaction has been fully detailed using thermogravimetry
in conjugation with evolved gas analysis, size exclusion chromatography and NMR spectroscopy.
For many applications poly(styrene) must
be treated to reduce its flammability. This is usually done by incorporating
a flame retardant additive, usually an organohalogen compound, into the formulation
as the polymer is being processed. A potentially very efficient way of introducing
flame retardance would be to incorporate a suitable structural unit directly
into the polymer. This can be done by using 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
as an initiator for styrene polymerization. The strained carbon–carbon
bond of the phospholane undergoes homolysis at moderate temperatures to generate
a diradical which initiates polymerization. The resulting polymer contains
an O–P–O unit in the mainchain. Thermogravimetry indicates that
the thermal stability of the polymer is quite comparable to that of poly(styrene)
generated by conventional methods.
Vinylidene chloride copolymers have a number of superior properties, most notably, a high barrier to the transport of oxygen
and other small molecules. As a consequence, these materials have assumed a position of prominence in the packaging industry.
At processing temperatures these copolymers tend to undergo degradative dehydrochlorination. The dehydrochlorination reaction
is a typical chain process with distinct initiation, propagation, and termination phases. It has been demonstrated that initiation
of degradation is strongly facilitated by the presence of unsaturation along the backbone. Such unsaturation may be introduced
via interaction of the polymer with a variety of agents which might commonly be encountered during polymerization or processing.
The presence of an unsaturated unit within the polymer generates an allylic dichloromethylene which may function as a major
defect (labile) site for the initiation of degradation. The conversion of these dichloromethylene units into non-reactive
groups would interrupt propagation of the dehydrochlorination reaction and lead to the stabilization of the copolymer. Potential
stabilization in the presence of metal formates has been examined using a vinylidene chloride/methyl acrylate (five mole percent)
copolymer and thermogravimetric techniques. The effect of the metal formate on the stability of the polymer reflects the relative
halogenophilicity of the metal cation present. Metal formates (sodium, calcium, nickel(II) and to a lesser extent lead(II),
cadmium, manganese(II) and magnesium) may be expected to be ineffective as stabilizers for vinylidene chloride copolymers.
At the other extreme, metal formates which contain cations sufficiently acidic to actively strip chlorine from the polymer
backbone, e.g., zinc formate, will function to enhance the degradation process. An effective carboxylate stabilizer must contain
a metal cation sufficiently acidic to interact with allylic chlorine and to facilitate its displacement by the carboxylate
anion. Copper(II) formate may possess the balance of cation acidity and carboxylate activity to function as an effective stabilizer
for vinylidene chloride copolymers.
Organohalogen flame retardants, particularly brominated aromatics, are popular, effective, low cost, and widely used in the
plastics industry. However, an increasing concern about persistence in the environment and potential negative health effects
of these materials has generated intense interest in the development of alternatives. Ideally, these should have all the positive
attributes of the materials that will be replaced. In addition, it is desirable that the new materials be as “green” as possible,
e.g., based on renewable resources and be degradable to nontoxic products in the environment. A series of new, non-halogenated
flame retardants based on tartaric acid is being developed. Tartaric acid is a by-product of the wine industry and is readily
available locally on an annual basis (Michigan is the thirteenth largest producer of wine in the U.S.). It can be readily
converted to the corresponding diethyl ester. This ester may serve as the base for the development of a series of new, non-halogenated
flame-retarding agents. The presence of the reactive hydroxyl groups allows the introduction of a variety of phosphorus-containing
moieties. For example, treatment of diethyl tartrate with diphenylphosphinyl chloride generates diethyl 2,3-di(diphenylphosphinato)-1,4-butanedioate.
This material may serve as a monomer for the preparation of various phosphorus-containing polymers and oligomers via step-growth
transesterification. The thermal stability of this compound has been assessed by thermogravimetry.
Vinylidene chloride polymers are prominent in the
barrier plastics packaging industry. They display good barrier to the transport
of oxygen (to prevent spoilage of food items) and flavor and aroma constituents
(to prevent 'scalping' on the supermarket shelf). However, these
polymers undergo thermal dehydrochlorination during processing. This can lead
to a variety of problems including the evolution of hydrogen chloride which
must be scavenged to prevent its interaction with the metallic walls of process
equipment. Such interaction leads to the formation of metal halides which
act as Lewis acids to facilitate the degradation. A potentially effective
means to capture hydrogen chloride generated might be to incorporate into
the polymer a mild organic base. Accordingly, copolymers of vinylidene chloride
and 4-vinylpyridine have been prepared and subjected to thermal aging. Results
suggest that the pyridine moiety is sufficiently basic to actively promote
dehydrochlorination in the vinylidene chloride segments of the polymer.
Vinylidene chloride copolymers are prominent in the barrier plastic packaging industry. These materials display excellent
barrier to the transport of oxygen (and other small molecules) as well as flavor and aroma molecules. However, they suffer
from a propensity to undergo degradative dehydrochlorination at process temperatures. To scavenge hydrogen chloride formed
and prevent its interaction with the metallic components of process equipment, a passive base is usually included as an additive
prior to processing. The base is most often an inorganic oxide or salt. These may negatively impact the properties of the
polymer, particularly as a film. An organic base that could be covalently incorporated into the copolymer might display better
behavior. Accordingly, a series of copolymers containing low levels of 4-vinylpyridine (0.05–3 mole%) have been prepared,
characterized, and examined by thermogravimetry to assess thermal stability. In all cases, polymers containing 4-vinylpyridine
units are less stable than the polymer containing none of this comonomer. Clearly, the pyridine moiety is a sufficiently strong
base to promote E2 elimination of hydrogen chloride to generate dichlormethylene units in the mainchain from which thermal
degradation may be initiated.
dynamic methods for estimating activation energies have been developed. This
development has arisen largely as a matter of convenience and the desire to
minimize analysis time. While these methods generally afford values which
are somewhat similar, the agreement among values from various methods is never
outstanding. Further, the values obtained are often, at best, only approximations
of the values obtained by the traditional isothermal approach. To better ascertain
the utility of dynamic methods for the determination of activation energies,
the activation energy for the thermal degradation of a standard vinylidene
chloride/methyl acrylate (five-mole percent) copolymer has been generated
by a variety of methods. The degradation of this polymer is an ideal reaction
for evaluation of the various methods. At modest temperatures (<200C),
the only reaction that contributes to mass loss is the first order evolution
of hydrogen chloride, i.e., there is only one significant reaction occurring
and it is not impacted by competing processes. The best values (most reproducible;
best correspondence to values obtained by titrimetry and other methods) are
those obtained by plotting the natural logarithm of rate constants obtained
at various temperatures vs. the reciprocal of the Kelvin temperature. Various
dynamic methods yield values which are less reproducible and which approximate
these values to a greater or lesser degree. In no case is the agreement good.
General purpose poly(styrene) prepared by conventional radical techniques contains a head-to-head unit as a consequence of
polymerization termination by radical coupling. As has been previously demonstrated, thermal stress promotes homolysis of
the bond linking the head-to-head components. The macroradicals generated depolymerize rapidly to generate styrene monomer.
This decomposition during processing can lead to finished articles containing objectionable levels of styrene monomer, particularly
for food packaging applications in which even low levels of monomer can promote objectionable taste and aroma. Polymer containing
no head-to-head units should not be prone to this facile decomposition. In this instance, poly(styrene) has been prepared
by nitroxyl-mediated polymerization of styrene monomer followed by reductive removal of nitroxyl end groups. Polymer prepared
in this manner contains no head-to-head units and displays thermal stability much greater than that observed for conventional
poly(styrene). A direct comparison of the stability for the two polymers is readily available by thermogravimetric techniques.
A quantitative reflection of the difference in stability is available from the rate constants for the respective decomposition.