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SHAPE-MEMORY
POLYMER DEVICE
DESIGN
PLASTICS DESIGN LIBRARY (PDL) PDL
HANDBOOK SERIES
Series Editor: Sina Ebnesajjad, PhD (sina@FluoroConsultants.com)
President, FluoroConsultants Group, LLC
Chadds Ford, PA, USA
www.FluoroConsultants.com
The PDL Handbook Series is aimed at a wide range of engineers and other professionals working
in the plastics industry, and related sectors using plastics and adhesives.
PDL is a series of data books, reference works and practical guides covering plastics engineering,
applications, processing, and manufacturing, and applied aspects of polymer science, elastomers and
adhesives.
No part of this publication may be reproduced or transmitted in any form or by any means, electronic or
mechanical, including photocopying, recording, or any information storage and retrieval system, without
permission in writing from the publisher. Details on how to seek permission, further information about the
Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance
Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.
This book and the individual contributions contained in it are protected under copyright by the Publisher
(other than as may be noted herein).
Notices
Knowledge and best practice in this field are constantly changing. As new research and experience broaden our
understanding, changes in research methods, professional practices, or medical treatment may become necessary.
Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any
information, methods, compounds, or experiments described herein. In using such information or methods they
should be mindful of their own safety and the safety of others, including parties for whom they have a professional
responsibility.
To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability
for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise,
or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.
ISBN: 978-0-323-37797-3
Ken Gall
Duke University, Durham, NC, United States
Jack C. Griffis
MedShape, Inc., Atlanta, GA, United States
Drew W. Hanzon
University of Colorado Denver, Denver, CO, United States
Stephen L. Laffoon
Vertera Spine, Inc., Atlanta, GA, United States
David L. Safranski
MedShape, Inc., Atlanta, GA, United States
Dalton G. Sycks
Duke University, Durham, NC, United States
Rui Xiao
Hohai University, Nanjing, China
Christopher M. Yakacki
University of Colorado Denver, Denver, CO, United States
Kai Yu
University of Colorado Denver, Denver, CO, United States
Cheng Zhang
Hohai University, Nanjing, China
ix
BIOGRAPHIES
xi
PREFACE
David Safranski and Jack Griffis started their work with shape-memory
polymers in 2005 and 2007, respectively. Each of them began in collabora-
tion with Ken Gall, who currently serves as chair of Mechanical
Engineering and Materials Science at Duke University. The Gall group
moved from the University of Colorado Boulder to Georgia Tech in
the summer of 2005, and David joined as an undergraduate assistant.
Owing to the jovial, high energy, start-up atmosphere, he continued his
graduate research with the Gall group. As MedShape grew out of the Gall
group, Jack joined MedShape as full-time employee number 2 and was
responsible for the US FDA clearance of shape-memory polymer devices
WedgeLoc, Morphix, ExoShape and Eclipse in 2008, 2009, 2011, and
2013 respectively. Following completion of his PhD in Materials Science
and Engineering, David joined MedShape in 2011. Together, David and
Jack have worked on numerous product development efforts, federally
funded research grants, and private contract research projects, all focused
on shape-memory and functional polymer technologies.
First and foremost, this book is meant to help you design a shape-
memory polymer device. We’ve organized this book in a rational
manner to help you accomplish this goal. In Chapter 1, Introduction
to Shape-Memory Polymers, we cover the basics of shape-memory
polymers, including thermomechanical properties, how to classify them,
fundamental mechanism, some limitations, and terminology. While ter-
minology may often be overlooked, it is critical to understand the
literature, especially when various authors have used different terms
over the past 20 years. We hope that Chapter 1, Introduction to Shape-
Memory Polymers, in particular, provides some consistency and clarity
when beginning to navigate design utilizing these functional materials.
In Chapter 2, Design, the design cycle involving shape-memory polymers
is described in detail. In addition, practical methods of manufacturing,
programming, and activation are covered along with environmental
considerations commonly overlooked in design, but that are critical
to these specialty materials. Chapter 3, Material Selection, serves as a
foundation for material selection by providing material selection case
studies of shape-memory polymer applications and tabulated properties
of a variety of shape-memory polymers. Chapter 4, Programming of
xiii
xiv Preface
We would like to thank all the contributors for their work and for their
assistance in reviewing sections of this book: Stephen Laffoon, Dalton
Sycks, Ken Gall, Rui Xiao, Cheng Zhang, Wei Min Huang, Drew
Hanzon, Kai Yu, and Christopher Yakacki. For their motivation and
support of this project, we would like to thank Anna Valutkevich,
Heather Cain, and Matthew Deans from Elsevier. We would like to
recognize all the members of the shape-memory polymer field for
publishing their work, which makes this book possible. We thank the
following companies that provided illustrations and figures of their
products: Composite Technology Development, Inc., EndoShape, Inc.,
HRL Laboratories, LLC., MedShape, Inc., and Spintech, LLC. We are
especially grateful to our wives Kathryn Safranski and Caroline Griffis for
their endless support, patience, and encouragement.
xv
CHAPTER ONE
Introduction to Shape-Memory
Polymers
David L. Safranski
MedShape, Inc., Atlanta, GA, United States
Contents
1.1 Introduction 1
1.2 History of Shape-Memory Polymers 3
1.3 Cross-linking, Thermal Transitions, and Shape-Memory Polymer Classification 5
1.3.1 Cross-linking 5
1.3.2 Thermal transitions 6
1.3.3 Classification of shape-memory polymers 8
1.4 Mechanism of the Shape-Memory Effect 8
1.5 Recovery Methods 11
1.6 Shape-Memory Polymer Terminology 13
1.7 Limitations of Shape-Memory Polymers 16
1.7.1 Recovery time and activation methods 16
1.7.2 Recovery force and work capacity 17
1.8 Overview of This Work 19
References 20
1.1 INTRODUCTION
Shape-memory polymers are a class of mechanically active polymers
that are able to change shape in response to a stimulus. They “memorize”
a permanent or original shape, undergo deformation to store a temporary
shape, and then return to their original “memorized” shape upon expo-
sure to a stimulus. A variety of stimuli may be used for actuation, such as
heat, infrared or visible light, solvents, magnetic fields, current, and
mechanical force [17]. The classic one-way shape-memory cycle has
three parts: (1) “programming,” which is the stage when the shape-
memory polymer is deformed from its original state into its temporary
shape using a mechanical force often at an elevated temperature;
(2) “storage,” which is the stage when the temporary shape is locked by
cooling below the activation temperature and removal of the mechanical
force; and (3) “recovery,” which is the stage when the shape-memory
polymer recovers or attempts to recover its original shape. Fig. 1.1 shows
the classic one-way shape-memory cycle. An object is heated above an
activation temperature, the object is deformed in some manner at this ele-
vated temperature, constrained at this elevated temperature in this new
temporary shape, then cooled to store the temporary shape. If released
from constraint, then heated, it will recover back to its original shape. If
constrained when heated, it will apply a force instead of recovering its
original shape. The majority of shape-memory polymers undergo so-
called one-way activation, when once the shape-memory polymer is
exposed to its stimulus, it returns to its original shape and remains in it. It
does not change back to its temporary shape upon cooling, it stays in its
permanent shape after recovery and any subsequent cooling.
Most current applications of shape-memory polymers use the one-way
activation between the temporary and original shapes. There are more
advanced chemistries and programming methods that allow for multi-
shape recovery and two-way shape-memory cycle, which will be discussed
later in Chapters 4 and 5, Programming of Shape Memory Polymers: The
Temperature Memory Effect and Triple/Multiple-Shape Memory Effect
in Polymers and Activation Mechanisms of Shape-Memory Polymers.
Figure 1.1 One-way shape-memory cycle with three steps: (1) Programming,
(2) storage, (3a) unconstrained recovery or (3b) constrained recovery. Adapted with
permission from RSC, Ortega, A.M., et al., Effect of cross-linking and long-term storage
on the shape-memory behavior of (meth)acrylate-based shape-memory polymers. Soft
Matter 2012;8(28): 7381.
Introduction to Shape-Memory Polymers 3
with elastic memory were also patented at this time that would allow for
blind riveting [14]. While not termed shape-memory polymers, the same
thermo-mechanical cycle was being utilized. Meanwhile, the mechanics
and thermodynamic principles behind rubber elasticity were being
developed in the mid-20th century by Guth, Treloar, and Flory [1517].
With the development of polymer cross-linking via irradiation,
Charlesby described the memory phenomenon as “an interesting and
often amusing property of lightly irradiated polyethylene” [18]. The first
major application of shape-memory polymers was heat-shrink tubing by
Paul Cook at RayChem Company in the late 1950s [19,20]. RayChem
initially started with cross-linking polyethylene, but now many polymers,
such as polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and
polyvinylidene fluoride (PVDF) are available as heat-shrink tubing. At
the same time, George Odian and colleagues at Radiation Applications,
Inc. developed deployable space structures from cross-linked polyethylene
in the early 1960s for NASA [21,22]. While the terminology in their
studies may have been different, they explored some of the critical aspects
of the shape-memory cycle. At that time, the term “elastic memory” or
“memory effect” was used to describe the recovery process along with a
“crystalline clutch mechanism” was used to describe shape fixity, but the
inherent process was the same as today [21,23]. A polymer, lightly cross-
linked semi-crystalline polyethylene, was deformed above the melting
temperature of the polyethylene crystals. The deformed part was cooled
into a new stable configuration because of recrystallization locking the
new shape, and the part could return to its original geometry upon heat-
ing. Fig. 1.2 shows the effect of radiation dosage and recovery tempera-
ture on the recovery of cross-linked polyethylene that was programmed
with a 180 bend at 80 C [21]. At the time, it was known that cross-links
and melting/recrystallization transition were involved in this ability to
hold a temporary shape and recover the original shape.
Over the course of the next several decades, the “elastic memory”
term from the field of radiation cross-linked polymers was still being used
to describe this phenomenon. Unfortunately, few research studies were
performed in this area after the initial studies in the 1960s, but industrial
applications were being explored as evidenced by the patent literature on
“heat-shrinkable” or “heat-recoverable” products and methods of joining
plastics. During the 1960s, NiTi, also known as Nitinol from Nickel
Titanium Naval Ordnance Laboratory, was gaining in popularity as well
as the term “shape-memory effect” to describe the behavior of NiTi and
Introduction to Shape-Memory Polymers 5
160 0 MRADS
12 MRADS
Deformation angle (°) 61 MRADS
80
40
0
20 40 60 80 100 120 140
Restoration temperature (°C)
Figure 1.2 Effect of cross-linking radiation dose on shape-recovery (as measured
from a 180 bend) of cross-linked polyethylene. Adapted with permission from Odian,
G. and B.S. Bernstein, The Use of Radiation-Induced Plastic Memory to Develop New
Space Erectable Structures. 1963, NASA: NY.
similar mechanically active alloys [24]. In the late 1980s and early 1990s,
researchers in Japan, especially at Zeon, Co., Asahi, Co., and Mitsubishi
Heavy Industries, Ltd. explored shape-memory polymers for a variety of
commercial applications using polynorbornenes and polyurethanes
[2528]. It was during this time that the use of the term “shape-mem-
ory” was used to describe polymers with “elastic memory,” even though
the inherent mechanisms of shape change are distinctly different between
shape-memory alloys and shape-memory polymers. Starting in the mid-
1990s, academic interest in this field dramatically grew, and started to
focus on the underlying mechanics primarily for both biomedical and
aerospace applications.
Figure 1.3 Schematic of polymer structures. (A) Linear, (B) branched, (C) lightly
cross-linked, (D) highly cross-linked.
glassy state to a flexible rubbery state as they are heated. Below the glass tran-
sition temperature, the polymer structure is glassy and rigid with limited
molecular motion. Above the glass transition temperature, the polymer
structure is mobile and large-scale molecular motion is possible. The glass
transition is often measured with three techniques: Differential scanning cal-
orimetry (DSC), dynamic mechanical analysis (DMA), and thermomechani-
cal analysis (TMA). For DSC, a step change occurs in the heat capacity of
the polymer during the glass transition. For DMA, a dramatic decrease in
storage modulus signifies the onset of the glass transition or the peak of the
tan delta is often used to represent the glass transition temperature, even
though the glass transition occurs over a temperature range. For TMA, a
change in volume or a change in the coefficient of thermal expansion occurs
when heating through the glass transition. For shape-memory polymers, this
rapid increase in viscosity (B1012 Pa s) and modulus during cooling
through the glass transition serves to lock in the temporary shape. The glass
transition temperature for shape-memory polymers can vary widely depend-
ing upon chemical structure and compositions, where the glass transition of
(meth)acrylates ranges from 223 C to 112 C [34]. Further discussion of the
theory of the glass transition can be found elsewhere [33,35,36].
The crystallization of polymer chains can also be used in the shape-
memory cycle. During cooling from the melt, some polymers organize
into crystalline lamellae, which are stacked polymer chains folded upon
themselves. These lamellae may further organize into larger crystalline
spherulites. While a portion of the polymer chains are stacked and folded
in these lamellae, a portion does not stack and remains in the amorphous
region outside of the crystalline lamellae. Thus, polymers that contain
both amorphous and crystalline regions are considered semi-crystalline.
During crystallization, chain mobility is restricted, which allows for a
temporary shape to be programmed using the shape-memory effect. This
is often accompanied by a large increase in modulus as the polymer cools
through this transition [35]. Conversely, the melt transition occurs when
polymer spherulites and lamellae lose their ordered stacking when heated
and return to the disordered melt. This allows for large-scale motion of
the polymer chains and the shape-memory polymer to return to its origi-
nal shape. Similar to the glass transition, DSC is widely used to character-
ize the crystallization and melt transitions. The crystallization transition is
given by an exothermic peak and the melt transition by an endothermic
peak during a DSC scan. Further discussion of the polymer crystallization
and melting can be found elsewhere [35,36].
8 Shape-Memory Polymer Device Design
Figure 1.4 Definition of four types of shape-memory polymers with different shape-
fixing and shape-recovery mechanisms depicted as a function of their dynamic
mechanical behavior. Plotted is the tensile storage modulus versus temperature as
measured using a small oscillatory deformation at 1 Hz for: (I) chemically cross-linked
glassy thermosets; (II) chemically cross-linked semi-crystalline rubbers; (III) physically
cross-linked thermoplastics; and (IV) physically cross-linked block copolymers. Reused
with permission by RSC, Liu, C., H. Qin, and P.T. Mather, Review of progress in shape-
memory polymers. J Mater Chem 2007; 17(16): 154358.
transition can be a melt transition, where the crystallites lock the tempo-
rary shape by preventing the polymer chains from returning to the origi-
nal permanent shape. Examples of this mechanism for three types of
polymers with two types of transition temperature and structures are
shown in Fig. 1.5 [38].
Figure 1.5 Schematic representation of the molecular mechanism of the thermally
induced shape-memory effect for (A) a multiblock copolymer with Ttrans 5 Tm, (B) a
covalently cross-linked polymer with Ttrans 5 Tm, (C) a covalently cross-linked
polymer with Ttrans 5 Tg. If the increase in temperature is higher than Ttrans of the
switching segments, these segments are flexible and the polymer can be deformed
elastically. The temporary shape is fixed by cooling down below Ttrans. If the polymer
is heated up again, the permanent shape is recovered. Used with permission from
Wiley, Lendlein, A. and S. Kelch, Shape-memory polymers. Angew Chem Int Ed, 2002;41:
203457.
Introduction to Shape-Memory Polymers 11