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Transcript of Polymers
I.INTRODUCTION
Polymer
A polymer is a chemical compound or mixture of compounds consisting
of repeating structural units created through a process
of polymerization. The term derives from the ancient Greek word πολύς
(polus, meaning "many, much") and μέρος (meros, meaning "parts"), and
refers to a molecule whose structure is composed of multiple repeating units,
from which originates a characteristic of high relative molecular mass and
attendant properties. The units composing polymers derive, actually or
conceptually, from molecules of low relative molecular mass. The term was
coined in 1833 by Jöns Jacob Berzelius, though with a definition distinct from
the modern IUPAC definition. Polymers are studied in the fields
of biophysics andmacromolecular science, and polymer science (which
includes polymer chemistry and polymer physics).
Historically, products arising from the linkage of repeating units
by covalent chemical bonds have been the primary focus of polymer science;
emerging important areas of the science now focus on non-covalent links.
Because of the stipulation as to repeating substructures, polymers are
formally a subclass of the category of macromolecules;
the polyisoprene of latex rubber and the polystyrene of styrofoam are
examples of polymeric natural/biological and synthetic polymers,
respectively. In biological contexts, essentially all biological macromolecules
—i.e., proteins (polyamides), nucleic acids (polynucleotides), and
polysaccharides—are purely polymeric, or are composed in large part of
polymeric components—e.g., isoprenylated/lipid-modified glycoproteins,
where small lipidic molecule and oligosaccharide modifications occur on the
polyamide backbone of the protein.
Hence, the terms polymer and polymeric material encompass very
large, broad classes of compounds, both natural and synthetic, with a wide
variety of properties. Because of the extraordinary range of properties of
polymeric materials, they play essential and ubiquitous roles in everyday
life, from those of familiar synthetic plastics and other materials of day-to-
day work and home life, to the natural biopolymers that are fundamental to
biological structure and function.
Polymers are large molecules composed of many similar smaller
molecules linked together. The individual smaller molecules are called
monomers. When small organic molecules are joined together, giant
molecules are produced. These giant molecules are known as
macromolecules.
Generally speaking, all macromolecules are produced from a small set
of about 50 monomers. Different macromolecules vary because of the
arrangement of these monomers. By varying the sequence, an incredibly
large variety of macromolecules can be produced. While polymers are
responsible for the molecular "uniqueness" of an organism, the common
monomers mentioned above are nearly universal.
II. DISCUSSION
A. Common Examples of Polymers
Natural polymeric materials such as shellac, amber, wool, silk and
natural rubber have been used for centuries. A variety of other natural
polymers exist, such as cellulose, which is the main constituent of wood and
paper. The list of synthetic polymers includes synthetic rubber, phenol
formaldehyde resin (or Bakelite), neoprene, nylon, polyvinyl chloride (PVC or
vinyl), polystyrene, polyethylene, polypropylene, polyacrylonitrile, PVB,
silicone, and many more.
Most commonly, the continuously linked backbone of a polymer used
for the preparation of plastics consists mainly of carbon atoms. A simple
example is polyethylene ('polythene' in British English), whose repeating unit
is based on ethylene monomer. However, other structures do exist; for
example, elements such as silicon form familiar materials such as silicones,
examples being Silly Putty and waterproof plumbing sealant. Oxygen is also
commonly present in polymer backbones, such as those of polyethylene
glycol, polysaccharides (in glycosidic bonds), and DNA (in phosphodiester
bonds).
B. Importance of Polymer Properties
Because of their high molecular mass, polymers, as compared to small
molecules, have unique properties that are often difficult to predict. As
such, some background knowledge of the physical chemistry of polymers is
desirable for dealing with polymers and polymeric materials.
Polymer properties, like solubility behavior, are used as a guide on a
laboratory scale when analyzing or characterizing polymers or when
determining structure-property relationships. On an industrial scale,
properties, such as melt viscosity or heat capacity, are important for
establishing polymerization and processing conditions. A listing of
properties is required for selecting polymers to meet specific applications.
Polymers are ubiquitous as they are used in all applications, from
consumer products to high-temperature industrial use to medical devices,
under a wide-range of conditions. In modern polymer science and
engineering, more complex structures, such as multilayer films,
nanomaterial, electro-optical and electronic devices are being developed
that require more specialized and complex testing for end-use performance
evaluation. Furthermore, from knowledge of structure-property relations of
polymers and polymeric materials, one can begin to design and tailor make
polymers and complex polymeric structures to meet specific end-use
performance requirements.
It is sometimes difficult to accurately predict end-use performance
characteristics of the final product using tabulated data of individual
components. As a result, accurate measurements are those made on the
final product itself, rather than using model polymers or components. In
these cases, empirically derived measurements using the actual product,
verified with authentic samples, may be the best option. It should be noted
that most empirically derived data are trade secrets, and, as such, not
available. Nevertheless, compilations of properties are still valuable.
C. Polymer Complexity
Because of polymer complexity, property variability must be taken into
consideration. In this section, we will discuss possible sources of polymer
inconsistency and offer suggestions to recognize and reduce these errors.
Chemical or compositional heterogeneity refers to the chemical or
structural difference among chains of the same polymer. Thus a measured
property of a chemically heterogeneous sample will be an averaged value
dependent upon sample source. For chemically homogeneous samples,
property variability will not be a concern. In a similar fashion, polymers that
are polydisperse in molecular weight have averaged property values,
while monodisperse samples will give accurate data. Obviously, samples
that are both chemically homogeneous and monodisperse will give the
most accurate and precise values.
As compared to synthetic polymers, almost all nucleic acids and
mammalian proteins are compositionally (chemically) homogeneous
and monodisperse, if not there would be no life; biopolymers carry highly
specific and selective information. Mammalian polysaccharides, for the
most part, are also compositionally homogeneous, but are polydisperse in
molecular weight; whereas plant polysaccharides are polydisperse.
Chemically modified cellulose (cellulosics) are typically both compositionally
heterogeneous and polydisperse in molecular weight. Starches (α-amylose
and amylopectin), another major class of polysaccharides, are highly
polydisperse in molecular weight, but quite compositionally homogeneous.
In addition, amylopectin and many other polysaccharides are highly
branched, which may further complicate listed property values.
Synthetic polymers can be quite complex and, as such, tabulated and
measured property data must be interpreted with
care. Homogeneous synthetic polymers are those produced from
condensation polymerization reactions, in which all polymer chains
are chemically indistinguishable from another. Even though these types of
polymers show a finite polydispersity of two, accuracy and precision will not
be compromised since all samples (and reference standards) will have the
same degree of polydispersity. Lastly, synthetic polymers produced by
addition polymerization (i.e., ionic, complex coordination catalytic, or free-
radical copolymerization), will have the greatest amount of compositional
heterogeneity, and with the exception of anionically polymerized samples,
will also have a large molecular weight polydispersity. For these polymers,
tabulated data must be interpreted with caution, unless users establish
their own data sets with reference polymers obtained from the same
polymerization conditions.
Sequence distribution or polymer microstructure is the next higher
level of complexity in which the average arrangement of monomers along a
chain is considered. The polymerization mechanism and reactivity ratios of
monomers dictate this parameter. Monomers can be randomly arranged
along chains in the case of statistic or random copolymers or in the
extreme form a block copolymers. In any event, the microstructure of
reference polymers should be defined when properties are listed.
Next in line of complexity is macromolecular architecture, or polymer
configuration, in which the topological nature of the chain is of interest.
Thus polymer branching can take on a wide range of configurations
including short- and long-chain branching, and comb, star, and dendritic
structures with or without comonomer segregation or blockiness. Because
of the strong influence of polymer configuration on properties, this
parameter needs to be defined, and care taken when comparing tabulated
data to those of actual samples.
In summary, polymers may have up to two or more distributed
characteristics depending on the number of different monomers used in the
polymerization, the type of polymerization mechanism, and whether or not
the sample was fractionated during isolation. As a rough estimate, polymer
"complexity" increases exponentially with the number of distributive
properties, making it more difficult to measure accurate polymer properties.
Some polymers are modified after polymerization; however, this
process can be somewhat difficult to control. Because polymer chain
segments can influence the chemistry of a neighboring groups. Chemical
modifications are done mainly on cellulosics and other polysaccharides to
tailor-make specific property characteristics. Thus tabulated property data
given for cellulosics and polysaccharides represent average values of the
entire sample ensemble of polymer chains that differ in composition. To
complicate matters further, insoluble gels, comprised of three-dimensional
networks, may form if chains are allowed to chemically or physically (via
hydrogen bonding) react with one another, either during or after
polymerization.
Post-polymerization processes are also accomplished via vulcanization,
irradiation, or through the addition of a low molecular weight cross-linking
agent. The resulting polymer (i.e., rubber, elastomer, resin, or gel) in
essence, is one super or giant molecule approaching infinite molecular
weight. Theseviscoelastic materials have wonderful consumer, industrial,
and aerospace end-use applications when properly formulated.
The next level of polymer complexity is polymer blends and
multicomponent systems. To adjust the glass-transition temperature,
plasticizers are added, often times at high concentrations. To increase
polymer strength, reinforced polymeric materials are used that consist of
added inorganic material, the most common being carbon black or glass
fibers. Laminated structures are also produced for increased material
strength.
High-value added, specialty products with controlled molecular weight,
branching, or architecture are being developed for high-technology
industries, most notably electronic and optical devices, printing inks, and
coatings in the aerospace industry. Because of their specialized uses, most
of these polymeric materials are not listed in this compilation.
D. Regulatory Agencies
Most industries issue testing protocols and polymer property
specifications to the trade. To ensure uniformity, national regulatory
agencies have formed to deal with standardized methods and testing
approaches. In the United States, ASTM is the most prominent independent
agency supported by industry with about 100 test methods in place
specifically for polymers and polymeric materials. API specializes in the
development of procedures for petroleum products, some of which are
polymeric. In Britain, BSI is the key agency for testing, while in Europe, DIN
procedures are followed. Many of these agencies are overseen by ISO, a
federation of national regulatory bodies. (See Table 1 for complete names
and acronyms.)
Governmental departments of commerce, defense, and military are
also involved in issuing protocols and specifications. For example, the FDA
is responsible for establishing acceptable limits of extractable components
from polymeric materials in contact with food and drugs.
Table 1. Key agencies involved in
standardized testing of polymers and
polymeric materials under the umbrella of
ISO.
Abbreviatio
nOrganization
API American Petroleum Institute
ASTM American Society for Testing and Materials
BSI British Standards Institution
DIN Deutsches Institut fur Normung
FDA Food and Drug Agency
ISOInternational Organization for Standardization
*
*Global federation of national standards
bodies representing 100 countries.
D. Reference Polymers and Specialty Materials
Sources of reference polymer standards that can be used for
instrument calibration and validating methods are listed in Table 2. In the
United States NIST is responsible for distributing a number of well-
characterized polymer standards.
These standards have well-defined chemical composition and
molecular weight, and are also suitable for formulating materials for R&D.
All reference standards and polymeric material should come with
certificates of analysis. (Since water content in polymers, especially
hydrophilic ones and polysaccharides, may affect properties, it is advisable
to vacuum dry and properly store them to prevent moisture buildup and
degradation.)
Table 2. Sources of polymer standards used
for instrument calibration, method
development and verification, and
formulating R&D samples
American Polymer Standards Corp USA
Gearing Scientific Ltd UK
National Institute of Standards and Technology USA
Polymer Laboratories Varian UK
Polymer Source Inc Canada
Polymer Standards Service (PSS) Germany
Pressure Chemical Co. USA
Putus Macromolecular China
Sigma-Aldrich USA
Tosoh Corporation Japan
Waters Corporation USA
E. Polymer Properties
In this section we discuss and list polymer properties that are included
in data tables of this book. Some properties reviewed in this section are not
listed in this text, but they are included for completeness. Specific
properties for certain classes of polymers are not given, especially those
used for optical, electronic and magnetic devices.
Much of this section and the book's content is based on van Krevelen's
(1976) property schemes, with modification. His book should be consulted
for more detailed discussions. Other books of interest are listed at the end
of this chapter.
Basic information that characterizes polymers is listed in Table 3.
These properties can be estimated from the expected outcome of the
polymerization, measured, or calculated from group contributions (see van
Krevelen, 1976). Methods for measuring these properties can be found in
the reference list (for example, see Barth and Mays, 1991; Brady, 2003;
Wu, 1995). Some of the more important properties will be considered here.
The most useful average molecular weights are the number- (Mn),
weight- (Mw), and z-averages (Mz). These averages are easily determined
from the molecular weight distribution obtained using size exclusion
chromatography (Mori and Barth, 2001). Oftentimes just the viscosity-
average molecular weight (Mv) is available, which can be conveniently
determined from the measured intrinsic viscosity of the polymer in a given
solvent at a specified temperature using tabulated Mark-Houwink
coefficients. Alternatively, Mw can be determined from light scattering and
Mn from osmometry.
Table 3. Basic Polymer Information
Property measured Remarks
CAS registration number
Physical state at rt
Chemical composition of
repeat units
Structural formula of repeat
group
Comonomer molar ratios For copolymers
Molar substitution For cellulosics
Molecular weight of repeat
unit
Statistical average molecular
weights
Mn, Mv, Mw, Mz, and
polydispersity
Percent added inorganic or
carbon filler or plasticizer
Polymer additives used to
impart selected performance
Polymer additives, e.g.,
antioxidants, UV stabilizers,
etc.
Moisture level If applicable
Branching, degree
(frequency) and extent
(length)
Short- or long-chain
branching if applicable, as
estimated
Polymer architecture
(topology), other than linear
or branched, if applicable
graft, star, comb, or
dendritic
Crystallinity
Tacticity
Microstructure, i.e.,
monomer sequence
distribution
Block, random, or alternate
Toxicity and stability Should be determined or at
least estimated from
structure of corresponding
comonomers
Environmental impactMust be known or at least
estimated for safe disposal
Branching, molecular topology, and comonomer sequence distribution
along the chain are more difficult to estimate; these properties are best
estimated by the chemistry of the polymerization procedure, with support
from NMR measurements. Polymer toxicity and stability must be known or
at least estimated from functional group and comonomer chemistry. It
should be realized that polymer toxicity, to a first approximation, is lower,
than the corresponding comonomer toxicity; because of the low polymer
diffusion coefficient, macromolecules cannot readily pass through
biomembranes, thus have limited bioavailability.
The effect of molecular weight of a polymer in solution on its colligative
properties, summarized in Table 4, is a well-established phenomeon. These
properties are dependent on the number of macromolecules in solution,
independent on molecular weight and chemical composition. In fact, the
number-average molecular weight of a polymer can be determined by
measuring one of its colligative properties.
Table 4. Colligative Polymer Properties
Property measured Remarks
Freezing point depression MW dependent
Boiling point elevation MW dependent
Vapour pressure depression MW dependent
Osmotic pressure elevation MW dependent
Table 5 lists volumetric properties of polymers in the liquid or solid
state as a function of temperature; these properties are related to the
compactness of chains and the interaction of comonomers within and
among neighboring chains. These properties are more dependent on
chemical composition, than molecular weight. Volumetric properties also
depend on factors influenced by comonomer sequence distribution, such as
tacticity, branching, and polymer crystallinity.
Table 5. Volumetric Properties
Property measured Remarks
Specific volume (reciprocal
of specific density)Depends on polymer state
Molar volume (reciprocal of
molar density)Depends on polymer state
Specific thermal expansivity Depends on polymer state
Molar thermal expansivity Depends on polymer state
Specific melt expansivityApplicable to crystalline
polymers
Molar melt expansivityApplicable to crystalline
polymers
Table 6 lists thermodynamic and calorimetric attributes of a polymer,
while Table 7 deals with polymer solubility and cohesive energy. Except for
molar entropy, all these properties depend mainly of chemical composition,
rather than molecular weight. Furthermore, polymer crystallinity, in
addition to the chemical nature of a polymer, plays a major role in dictating
solubility behavior. In order to effect solubility in the case of crystalline or
semicrystalline polymers, the solution must be heated near or above its
melting point to break up crystalline regions.
Table 6. Calorimetric and Thermodynamic
Properties Including Transition Temperatures
Property measured Remarks
Molar entropy
Molar enthalpy
Molar heat capacity
Latent heat of crystallization
Thermal conductivity
Melting temperature, Tm
Disappearance of polymeric
crystalline phase
Glass-transition
temperature, Tg
Onset of extensive
macromolecular motion
Secondary transition
temperaturesOther than Tm and Tg
Deflection temperature (heat
distortion)
Highest continuous
temperature material will
withstand
Vicat softening pointTemperature at which a
needle penetrates material
Brittleness temperature
Table 7. Cohesive Properties and Solubilities
Property measured Remarks
Cohesive energy
Cohesive energy density
Related to the "internal
pressure" of a polymer in
solution
Surface and interfacial
energy
Solubility parameterEqual to the square root of
the cohesive energy density
Good Solvency
Good solvent imparts
solubility via polymer
solvation
NonsolvencyPoor solvent cannot solvate
polymer
Theta temperature
The temperature at which
polymer-polymer, polymer-
solvent, and solvent-solvent
interactions are equal
Theta solvent
A solvent in which polymer-
polymer, polymer-solvent,
and solvent-solvent
interactions are equal
Light scattering and inherent viscosity measurements made at infinite
dilution are used to determine polymer size parameters, conformation,
2nd virial coefficient, weight-average molecular weight, and long-chain
branching parameters (Table 8). These are fundamental parameters that
allow us to probe structural features of polymer molecules. These
properties are dependent on molecular mass and shape, rather than
polymer composition.
Table 8. Dilute Solution Properties
Property measured Remarks
Intrinsic viscosity
Measured quantity related to
the hydrodynamic shape and
molecular volume of a
polymer in solution
Mark-Houwink coefficients
Coefficients related to the
shape of macromolecules in
solution.
Molecular conformation Molecular shape parameter
Specific refractive index
Parameter needed for
calculating Mw from light
scattering data
Polymer-solvent 2nd virial
coefficient
Determined from light
scattering measurements
Radius of gyrationMacromolecular size
parameter
End-to-end distance Macromolecular size
parameter
Hydrodynamic volumeMacromolecular volume
parameter
Melt index and viscosity are critical parameters needed for polymer
processing. These and other polymer transport properties are listed in Table
9. As in the case of other viscosity measurements, these properties depend
mainly on higher statistical molecular weight averages, such as Mw and Mz.
Table 9. Transport Properties
Property measured Remarks
Melt viscosity
Depends on molecular
weight and chain
entanglement
Melt indexInversely proportional to
viscosity
Gas permeability across a
polymer film or membrane
Usually water vapor, oxygen,
nitrogen, or carbon dioxide,
or specialty gases
Diffusion coefficient
Diffusion of polymer in a
given solvent at defined
conditions
Water absorption Water content taken up at
specified relative humidity
and temperature
Tables 10 to 13 list polymer characteristics directly involved with end-
use properties: mechanical properties (Table 10), electric and magnetic
properties (Table 11), optical properties (Table 12), and polymer stability
(Table 13). (A more complete discussion of these properties is given in
selected references at the end of this chapter.)
Table 10. Mechanical Properties
Property measured Remarks
Adhesion (tackiness)
Ball indentation hardness
Bulk modulus (reciprocal of
compressibility)
Coefficient of friction
Compression strengthForce needed to rupture
material
Tensile creep Shape change of material
caused by suspended weight
DampingAbsorption or dissipation of
vibrations
Dynamic mechanical
behavior
Elastic modulus
Elongation
Fatigue Number of cycles required
for fracture
Flexural stiffness
Flexural strength at break Amount of stress needed to
break material
Fracture mechanical
properties
Fracture energy, fatigue
resistance, fatigue crack
growth, void coalescence
Friction abrasion and
resistance
Hardness Resistance to compression,
indentation, and scratch
Impact strength Energy absorbed by sample
prior to fracture
Indention hardness
Load deformation
Mar resistance
Mold shrinkage
Poisson's ratio
Scratch resistance
Shear strengthMaximum load to produce a
fracture by shearing
Surface abrasion resistance
Tear resistance
Tensile strength break
(yield)See Young's modulus
Toughness
Amount of energy to break a
material (area under stress-
strain curve)
Ultimate strength
Viscoelastic behavior
Young's modulus(Tensile
strength)
Modulus of elasticity or
tensile modulus
Table 11. Electrical and Magnetic Properties
Property measured Remarks
Arc Resistance
Time needed for current to
make material surface
conductive because of
carbonization
Dielectric constant
Ability of material to store
electric energy for capacitor
application
Dielectric permittivity
Dielectric strengthVoltage required to break
down or arc material
Dissipation power factor
(loss tangent)
Watts (power) lost in
material used as insulator
Insulation resistance
Magnetic susceptibility
Resistivity
Volume resistivity
Tables 10 to 13 list polymer characteristics directly involved with end-
use properties: mechanical properties (Table 10), electric and magnetic
properties (Table 11), optical properties (Table 12), and polymer stability
(Table 13). (A more complete discussion of these properties is given in
selected references at the end of this chapter.)
Table 12. Optical Properties
Property measured Remarks
Colour
Physiological response;
measured using three
parameters: lightness,
chroma, and delta
Luminous transmittanceMeasure of plastic haze or
clarity
Molar refraction
Percent transmission Transparency
Refractive index
Specular glossSurface "flatness"; mirror
"finish"
Total internal reflectanceUV-visible absorbance
spectrum
Table 13. Polymer Stability
Property measured Remarks
Accelerated aging studies
Biological stabilityStability in the presence of
microorganisms
Burning rate
Chemical resistance
Hydrolytic stability (extreme
pH conditions), exposure to
chemicals and solvents
Flammability Flame resistance
Flash ignition temperature
Long-term immersion
PermeabilityAmount of gas or liquid
penetrating film
Recyclability
Resistance to cold
Self-extinguishing
temperature
Stress cracking Caused by weathering
III.CRITIQUE
Polymers are made up of many many molecules all strung together to
form really long chains (and sometimes more complicated structures, too).
What makes polymers so fun is that how they act depends on what
kinds of molecules they're made up of and how they're put together. The
properties of anything made out of polymers really reflect what's going on at
the ultra-tiny (molecular) level. So, things that are made of polymers look,
feel, and act depending on how their atoms and molecules are connected, as
well as which ones we use to begin with! Some are rubbery, like a bouncy
ball, some are sticky and gooey, and some are hard and tough, like a
skateboard.
IV. CONCLUSIONS
Polymer science can be viewed as an applied branch of chemistry
based on deliverable properties. It is of interest to note that most of these
properties depend on just four attributes: 1. polymer molecular weight, 2.
crystallinity, 3. chemical composition, and 4. macromolecular topology or
architecture; furthermore, these parameters interact with one another in a
complex manner. By varying these parameters, polymers can be tailor-
made to fit a list of desirable characteristics. It is hoped that this polymer
property database will serve as a guideline to help pave the way for the
development of newer materials of improved characteristics.
V. REFERENCES
http://www.polymersdatabase.com/intro/index.jsp
http://en.wikipedia.org/wiki/Polymer
http://biology.about.com/od/molecularbiology/ss/polymers.htm
http://www.merriam-webster.com/dictionary/polymer
DON MARIANO MARCOS MEMORIAL STATE UNIVERSITY
MID-LA UNION CAMPUS
COLLEGE OF ENGINEERING
CITY OF SAN FERNANDO, LA UNION
REPORT IN
MATERIALS ENGINEERING LECTURE
“POLYMERS”
REPORTED BY:
MHAR JOHN G. GALON
JOSE T. CORPUZ
SUBMITTED TO:
ENGR. JULIUS RAUL C. SAMPAGA