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A Literature Review on the Study of Moisture in Polymers

TL;DR: In this paper, the main chemical and physical interactions between moisture and the polymer matrix are discussed, as well as specific examples of how moisture sorption influences polymers include natural fiber-polymer composites, starch-based biodegradable thermoplastics, and thermoset polyurethane and epoxies.
Abstract: This literature review covers the main chemical and physical interactions between moisture and the polymer matrix. Fickian versus Non-Fickian diffusion behaviors are discussed in approximating the characteristics of moisture sorption. Also, bound water and free water sorbed in polymers are distinguished. Methods to distinguish between bound and free water include differential scanning calorimetry, infrared spectroscopy, and time-domain nuclear magnetic resonance spectroscopy. The difference between moisture sorption and water sorption is considered, as well as the difficulties associated with preventing moisture sorption. Furthermore, specific examples of how moisture sorption influences polymers include natural fiber-polymer composites, starch-based biodegradable thermoplastics, and thermoset polyurethane and epoxies.

Summary (1 min read)

INTRODUCTION

  • Polymers are distinct from many materials, because at ambient temperatures, low molecular weight substances can freely migrate throughout the polymer structure.
  • Therefore there is a desire to develop methods of preventing chemical and physical interactions with low molecular weight substances, such as gases and moisture.
  • The amount and rate of moisture absorption is dependent on temperature, relative humidity (RH) levels, as well the relative hydrophilicity of the polymer matrix, and the moisture sorption mechanism.
  • In general, if the moisture exposure time occurs before polymer relaxation takes place, only Fickian transport is considered.
  • Bound water consists of water molecules which come into contact with the solid polymer.

CURRENT APPLICATIONS

  • As mentioned above, moisture content in polymers can be analyzed by differential scanning calorimetry and infrared spectroscopy.
  • The results shown in Table 1 reveal that moisture sorption and water sorption are different processes because the hydrophobic coating protects against water sorption, but has no effect on moisture sorption.
  • Additionally, high moisture content in fibers can result in porous products.
  • Studies by Shogren et al. and Soest et al. have determined that rates of aging increased with increased initial moisture content, particularly above 20% w/w. Starch-based thermoplastics are amorphous directly after processing.
  • 12 Bound water, pictured in the polyurethane shape memory polymer , reduces the glass transition temperature almost linearly as well as influencing the polymer’s tensile behavior.

FUTURE DIRECTIONS

  • Currently, the most widespread methods of determining polymer water content and the ratio of bound and free water in polymers are differential scanning calorimetry and infrared spectroscopy.
  • The method is a simple, rapid, and non-destructive.
  • The instrument allows the user to differentiate between bound and free water because they each have distinct relaxation times.
  • This is imperative for hydrophilic polymers, specifically those used in drug formulations or electronics packaging, where chemical changes or swelling due to bound water are unacceptable.

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LA-UR-16-23708
Approved for public release; distribution is unlimited.
Title: A Literature Review on the Study of Moisture in Polymers
Author(s): Trautschold, Olivia Carol
Intended for: Literature Review, Graduate Term Paper
Issued: 2016-05-25

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A Literature Review on the Study of Moisture in Polymers
Olivia C. Trautschold
Department of Chemistry, University of Oregon, 1585 E 13th Ave, Eugene, OR 97403
Los Alamos National Laboratory, Los Alamos, NM 87545
6 June 2016
ABSTRACT: This literature review covers the main chemical and physical interactions between moisture
and the polymer matrix. Fickian versus Non-Fickian diffusion behaviors are discussed in approximating
the characteristics of moisture sorption. Also, bound water and free water sorbed in polymers are
distinguished. Methods to distinguish between bound and free water include differential scanning
calorimetry, infrared spectroscopy, and time-domain nuclear magnetic resonance spectroscopy. The
difference between moisture sorption and water sorption is considered, as well as the difficulties
associated with preventing moisture sorption. Furthermore, specific examples of how moisture sorption
influences polymers include natural fiber-polymer composites, starch-based biodegradable
thermoplastics, and thermoset polyurethane and epoxies.

1
INTRODUCTION
Polymers are distinct from many materials, because at ambient temperatures, low
molecular weight substances can freely migrate throughout the polymer structure. Therefore there
is a desire to develop methods of preventing chemical and physical interactions with low molecular
weight substances, such as gases and moisture.
1
Moisture and water sorption in polymers are
particularly important to consider when optimizing drug formulations and electronics.
2,3
In the
case of drug formulations, it is crucial that the product remains chemically stable and performs
consistently in a broad range of temperatures and ambient humidity levels once it reaches the
consumer. Whereas polymers used in electronics, such as poly(methyl methacrylate) optical disk
substrates, may bend or swell after moisture sorption following melt molding. In this case,
problems often arise during production, prior to consumer interaction.
3
The amount and rate of moisture absorption is dependent on temperature, relative humidity
(RH) levels, as well the relative hydrophilicity of the polymer matrix, and the moisture sorption
mechanism.
4
One useful model to approximate the characteristics of moisture sorption is Fickian
transport.
Equation 1. General diffusion law or Fick's second law of diffusion.
4


= 󰇧

+

+

󰇨
Fickian transport is described by the general diffusion law, Equation 1. C is the moisture
concentration (kg/m
3
), t is the time (s), and D is the diffusion coefficient (m
2
/s). The temperature
dependence of moisture absorption is then described by Equation 2.
Equation 2. The Arrhenius equation.
4
=


In Equation 2, D
o
is a pre-factor, E
d
is the activation energy, k is Boltzmann’s constant and
T is the absolute temperature. Equation 1 assumes that the diffusion coefficient is independent of
moisture concentration. Additionally, Fickian diffusion does not account for polymer changes due
to relaxation or degradation. Non-Fickian behavior explains such polymer relaxation processes
and reactions between moisture and the polymer matrix, such as hydrogen bond formation.
4
In
general, if the moisture exposure time occurs before polymer relaxation takes place, only Fickian
transport is considered. The length of time in which one may consider only Fickian diffusion is
generally valid for less than ten days, dependent on the polymer and humidity levels.
4
Whereas if
moisture exposure time is long enough for the polymer to undergo relaxation, both Fickian and
Non-Fickian transport must be taken into account. Note in Figure 1, the first 72 hours in which the
sample is subjected to 60% relative humidity at 30°C are described by Fickian transport. The
remaining time must then take Non-Fickian transport into account.

2
Figure 1. Moisture weight gain curve for a 0.6mm bismaleimide-triazine core sample subjected
to 30°C/60% RH.
4
Absorbed water effects every polymer differently, however there are generally two types
of water found in polymers, free water and bound water. Free, or unbound, water does not come
in direct contact with the solid polymer matrix. Free water in polymers exhibits the same transition
temperature, enthalpy, and peak shape as pure water in differential scanning calorimetry (DSC).
5
This phenomenon is shown in Figure 2. Peak I, denoting free water content, increases with
increasing sorbed water in curve D. Additionally, free water molecules in solid polymers, denoted
by peak 8 in Figure 3, reveal the same vibration modes as pure water in infrared (IR) spectroscopy.
6
Bound water consists of water molecules which come into contact with the solid polymer. The
water molecules act to break the intermolecular hydrogen bonds by forming hydrogen bonds with
the hydroxyl groups of polymers.
5
In hydrophobic polymers, water will even bind to the polymer
by forming weak hydrogen bonds with atoms such as fluorine.
3
In DSC, bound water has a lower
transition temperature and a different peak shape than free water, as shown in Figure 2, where Peak
II denotes bound water.
5
Furthermore, the IR vibration mode for bound water, peak 7 in Figure 3,
appears at a lower frequency than that of free water.
6

Citations
More filters
Journal ArticleDOI
TL;DR: In this article, the effect of voids on the state of water absorbed in epoxy was examined using differential scanning calorimetric analysis for a range of void contents (0 to 50%).
Abstract: The effect of voids on the state of water absorbed in epoxy has been examined using differential scanning calorimetric analysis for a range of void contents (0 to 50%). For void-free specim...

13 citations

Journal ArticleDOI
TL;DR: In this paper, the influence of voids on the moisture uptake of epoxy matrix has been studied, and void geometry and content were analyzed using microscopy images of the voids.
Abstract: The influence of voids on the moisture uptake of epoxy matrix has been studied. Specimens with void contents from 0 to about 50% were prepared. Void geometry and content were analyzed using microsc...

9 citations


Cites background from "A Literature Review on the Study of..."

  • ...Water absorption depends on the polymer chemical structure, the surrounding environment (humid air or liquid water), and the temperature.(1,2) Many researchers have identified that the ingress water in polymers tends to degrade the material, manifested in reduced glass transition temperature and modulus, e....

    [...]

Frequently Asked Questions (2)
Q1. What have the authors contributed in "A literature review on the study of moisture in polymers" ?

This literature review covers the main chemical and physical interactions between moisture and the polymer matrix. Fickian versus Non-Fickian diffusion behaviors are discussed in approximating the characteristics of moisture sorption. Furthermore, specific examples of how moisture sorption influences polymers include natural fiber-polymer composites, starch-based biodegradable thermoplastics, and thermoset polyurethane and epoxies. 

Therefore future research should investigate the two sorption mechanisms and alternative methods to prevent both water and moisture sorption.